We are supposed to pray each of the five daily prayers at its appointed time. However, in exceptional circumstances, we are allowed to combine dhuhr with asr, and maghrib with isha. Therefore, if you have classes at the time of dhuhr and you cannot get a break from it until the time of 'asr, then you may pray dhuhr at the time of 'asr, followed by 'asr.
So, if you are faced with a situation where you cannot dhuhr on time, then you may combine it with 'asr at home. Otherwise, you ought to pray at the school.
For further details, pls consult the following answer.