Wa alaykum SAlam,
You can pray at any time before Asr begins, of course it is best to pray before that time.
Also, perhaps you should investigate getting a prayer room, or perhaps just use a classroom for 10 minutes, you can arrange that with your teachers inshaAllah.
It is your human right to do that under the Law.
When i was around 9 years old in primary school, i managed to get a letter and have Friday Afternoons off
and in Secondary school i would go to the mosque on Fridays,
at other times during week i could pray Zohr at the mosque, or i could pray in the school at the time. As i had arranged to have a room for prayers.