The school administration refuses to let students sit on chairs and says that sitting on the ground is Sunnah!

Sitting on the ground or sitting on chairs is not in and of
itself Sunnah or bid‘ah (innovation); rather that depends on people’s
customs in sitting and on the place where they are sitting. 

If they are siting in the mosque – for example – then the
hereditary custom in most of the mosques of the Muslims is to sit on the
floor. This is how the hadeeths describe the Prophet (blessings and peace of
Allah be upon him) and his Companions. Hence it is not prescribed for people
who are meeting in the mosque to sit on chairs, except for those who need to
do so, such as if the teacher or khateeb needs to be visible to the people,
in which case he may sit on a chair in order to teach them, which is also
the custom in the mosques of the Muslims. 

Or if anyone else in the gathering or who is praying needs to
sit on a chair because of sickness or the like (he may also sit on a chair
in the mosque). 

With regard to all other gatherings, it depends on what
people are accustomed to and what is easiest for them in those gatherings.
If it is more appropriate for them to sit on chairs, then there is nothing
wrong with that and it is not contrary to the Sunnah. Students at all stages
of education still sit on the benches that are made for them and we do not
know of any scholar who ever objected to that or enjoined sitting on the
floor because it is closer to the Sunnah. 

Al-Bukhaari (3238) and Muslim (161) narrated from Jaabir (may
Allah be pleased with him), in the story of the beginning of the Revelation,
that he heard the Prophet (blessings and peace of Allah be upon him) say:
“Then the Revelation ceased for a while, then whilst I was walking, I heard
a voice from heaven. I lifted my gaze to the heavens, and there was the
angel who had come to me in Hira’, sitting on a chair between heaven and
earth…” 

And Muslim (876) narrated from Abu Rifaa‘ah (may Allah be
pleased with him) who said: I came to the Prophet (blessings and peace of
Allah be upon him) when he was delivering a khutbah and I said: O Messenger
of Allaah, here is a stranger who has come to ask you about his religion,
for he does not know what his religion is. The Messenger of Allaah
(blessings and peace of Allah be upon him) turned to me and left his khutbah.
He came to me, and a chair was brought, I thought its legs were made of
iron. The Messenger of Allaah (blessings and peace of Allah be upon him) sat
on it and started telling me of what Allaah had told him, then he went back
and completed his khutbah. 

Abu Dawood (113) narrated that ‘Abd Khayr said: I saw ‘Ali
(may Allah be pleased with him) bring a chair and sit on it, then a vessel
of water was brought and he washed his hands three times, then he rinsed his
mouth and his nose with one (handful of) water … And he narrated the hadeeth. 

Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

The Standing Committee was asked: 

Is it true that eating at a table – dining room – is
imitation of the kuffaar, and is using a spoon or a fork whilst eating a
kind of arrogance or imitation of the kuffaar? 

The committee replied: There is nothing wrong with eating at
a table and the like or with eating with a fork and spoon and so on. That is
not imitating the kuffaar because these are not things that are unique to
them. 

End quote. Fataawa al-Lajnah al-Daa’imah, 26/309 

And Allah knows best.