They pray Jumu‘ah approximately one hour before the sun passes its zenith because of their classes

Praise
be to Allah

The majority of Hanafi, Maaliki and Shaafa‘i scholars are of
the view that Jumu‘ah prayer cannot be offered until after the sun has
passed the zenith, which is the beginning of the time for Zuhr prayer.

See: al-Umm (1/223); al-Majmoo‘ (4/377-381);
al-Mawsoo‘ah al-Fiqhiyyah (27/197-198)

They quoted as evidence the report narrated by Muslim (860)
from Salamah ibn al-Akwa‘ (may Allah be pleased with him) who said: We used
to pray Jumu‘ah with the Messenger of Allah (blessings and peace of Allah be
upon him) when the sun passed its zenith, then we would go back and try to
seek shade.

(They also quoted as evidence) the report narrated by al-Bukhaari
(904) from Anas ibn Maalik (may Allah be pleased with him) that the Prophet
(blessings and peace of Allah be upon him) used to pray Jumu‘ah when the sun
began to decline (after passing the zenith).

The Hanbalis are of the view that it is valid to pray Jumu‘ah
before the sun passes its zenith, and they quoted as evidence some hadiths
and reports from the early generation, the apparent meaning of which
indicates that it is permissible to pray Jumu‘ah before the sun passes its
zenith.

Muslim (858) narrated from Abu Ja‘far al-Baaqir that he asked
Jaabir ibn ‘Abdullah: When did the Messenger of Aleah (blessings and peace
of Allah be upon him) pray Jumu’ah? He said: He used to pray, then we would
go to our camels and let them rest, when the sun had passed its zenith.

This indicates that he prayed it before the sun passed its
zenith.

Al-Bukhaari (939) and Muslim (859) narrated that Sahl ibn
Sa‘d (may Allah be pleased with him) said: We used not to take a nap or eat
lunch until after Jumu‘ah at the time of the Messenger of Allah (blessings
and peace of Allah be upon him).

Ash-Shawkaani (may Allah have mercy on him) said:

This provides evidence for those who say that it is
permissible to pray Jumu‘ah  before the sun passes its zenith. That was the
view of Ahmad ibn Hanbal.

Their argument is that the time for lunch and the mid-day nap
is before the sun passes its zenith. They narrated from Ibn Qutaybah that he
said: It is not called lunch or mid-day nap after the sun has passed its
zenith.

Ibn al-‘Arabi had an odd view and narrated that there was
consensus that it does not become obligatory until the sun has passed its
zenith, apart from what was narrated from Ahmad, but this is to be rejected.
Ibn Qudaamah and others narrated from a number of the early generations a
view similar to that of Ahmad. End quote.

Nayl al-Awtaar
(3/319)

Ibn Abi Shaybah narrated in al-Musannaf (5140) from
Bilaal al-‘Absi that ‘Ammaar led the people in praying Jumu‘ah, when the
people were of two groups. Some of them said that the sun had passed the
zenith and some said that it had not done so.

Classed as saheeh by al-Albaani in al-Ajwibah an-Naafi‘ah
(p. 24)

The scholars of the Standing Committee for Issuing Fatwas
were asked: 

Is it permissible to pray Jumu‘ah one hour before the sun
passes the zenith – because of having to start work in France? Please note
that if we do not pray it before we start work, which is one hour before the
sun passes the zenith, we will not be able to pray Jumu‘ah. Does this case
of necessity make it permissible?

They replied:

There is a difference of scholarly opinion concerning the
definition of the beginning of the time for Jumu‘ah prayer. Most of the
fuqaha’ are of the view that the beginning of its time is the same as the
beginning of the time for Zuhr, which is when the sun passes the zenith, so
it is not permissible to pray it before the sun passes the zenith, whether
that is a long time before or a short time. That is not acceptable, because
of the words of Salamah ibn al-Akwa‘ (may Allah be pleased with him): We
used to pray Jumu‘ah with the Messenger of Allah (blessings and peace of
Allah be upon him) when the sun passed its zenith, then we would go back and
try to seek shade. Narrated by al-Bukhaari and Muslim. And because of the
words of Anas ibn Maalik (may Allah be pleased with him):

The Messenger of Allah (blessings and peace of Allah be upon
him) used to pray Jumu‘ah when the sun began to decline (after passing the
zenith). Narrated by al-Bukhaari. A number of scholars said: it is not
permissible before the sixth or fifth hour (i.e., approximately one of two
hours before the time of Zuhr prayer).

Imam Ahmad ibn Hanbal and a number of other scholars are of
the view that the beginning of its time is the beginning of the time for Eid
prayer; as for the time when the sun passes its zenith, that is the
beginning of the time when it becomes obligatory to hasten to do it. They
quoted as evidence for it being permissible to offer this prayer before the
sun passes the zenith the words of Jaabir (may Allah be pleased with him):
The Messenger of Allah (blessings and peace of Allah be upon him) used to
pray – i.e., Jumu‘ah – then we would go to our camels and let them rest,
when the sun had passed its zenith. Narrated by Abu Dawood.

We may reconcile between the hadiths by noting that the
Messenger of Allah (blessings and peace of Allah be upon him) used to pray
it after the sun had passed the zenith most of the time, but he prayed it
shortly before the sun passed the zenith on some occasions.

Based on that, it is better to pray it after the sun has
passed its zenith, following what the Prophet (blessings and peace of Allah
be upon him) used to do most of the time, and so as to avoid an area of
scholarly differences of opinion. This is what indicates that this is a
matter of ijtihaad and that it is broad in scope.  But for those who pray it
shortly before the sun passes its zenith, their prayer is valid, in sha
Allah, especially if they have a valid excuse, such as that mentioned by the
questioner. End quote.

Fataawa al-Lajnah ad-Daa’imah
(8/216-217)

Shaykh Ibn Baaz (may Allah have mercy on him) was asked:

Is it permissible to pray Jumu‘ah before the sun passes the
zenith?

He replied: It is permissible to pray Jumu‘ah before the sun
passes the zenith, but it is preferable to pray it after the sun has passed
its zenith, so as to avoid a matter concerning which the scholars differed,
because most of the scholars say that Jumu‘ah prayer must be offered after
the sun has passed its zenith. This is the view of the majority, but some of
the scholars are of the view that it is permissible to offer it before the
sun passes its zenith, in the sixth hour, and there are saheeh hadiths and
reports which indicate this. So if someone offers this prayer shortly before
the sun passes its zenith, then his prayer is valid, but it should not be
done until after the sun has passed its zenith, so as to act in accordance
with all of the hadiths and so as to avoid a matter concerning which the
scholars differed, and so as to make it easy for all the people to attend
together, and so that the prayer may be offered at one time. This is what is
preferable and is more prudent. End quote.

Majmoo‘ Fataawa Ibn Baaz
(12/391)

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was
asked:

There are some khateebs who enter the mosque on Friday and
begin the khutbah ahead of time, and they may give the iqaamah (call
immediately preceding the prayer) when the sun has not yet passed its
zenith. How valid is that?

He (may Allah have mercy on him) replied:

There is a difference of opinion among the scholars
concerning this issue – i.e., beginning the khutbah and prayer on Friday
before the sun passes its zenith. Some of them say that it is not
permissible to do so until the sun has passed its zenith, and some of them
say that it is permissible. The correct view is that it is permissible one
hour or half an hour before the sun passes its zenith, or something close to
that, but it is preferable to do it after the sun has passed its zenith,
even according to those who say that it is permissible to bring it forward
by an hour or so.

If someone prays before the sun passes the zenith, based on
the opinion which says that doing so is permissible before the sun passes
its zenith, then there is nothing wrong with it, but it should be only a
short time before. End quote.

Liqa’ al-Baab al-Maftooh
(16/19)

Based on that, there is nothing wrong with you praying
Jumu‘ah approximately one hour before the beginning of the time for Zuhr,
but to be on the safe side you should not make it more than one hour. But if
it is possible to pray it after the sun has passed its zenith, then that is
preferable and is better.

And Allah knows best.