A school should not force you to do things in which you do not want to do for religious reasons. Talk to a member of staff to resolve this. If you try your best to resolve it and they still force you to do the activity, keep a low profile and don't engage in more than is needed of you, then Allahu a'lam if it counts as a sin. This is not your intention, though and in Islam, you intend that matters. There are ahadith and there are Quranic verses that support this, like:
“There are four types of people: one is a man whom Allah has given knowledge and wealth. He acts concerning his wealth based on his knowledge. Another person says that if Allah had given him similar to what He gave the first man, he would have acted in the same fashion. The reward for both of them will be the same. A third person is one, whom Allah gives wealth but He does not give knowledge. Therefore, he spends money according to his desire. Another man says that if Allah had given him, what He had given that person, he would have acted in the same manner. These two will have the same burden upon them.” [Ibn Majah with a good chain]
“Verily, all actions are but driven by intention and for everyone is what he intended.” [Bukhari and Muslim]
It tells us that even if you do not have the means to do something if you want to do it, it still counts as a haram / halal. If you are put in a situation like this, try your best to avoid the haram activities within (for example, drinking alcohol in a celebration or eating pork in a restaurant etc.)
If you are helpless, then it would not count against you. Allah is the most just being that can ever exist, and he would not put you in such a situation and force a sin to be recorded if you cannot help it. That is morally wrong, holding somebody accountable for something they can't help.
Allah knows best. Have a nice day.