Is it permissible to remain in a non-Islamic school that does not teach Christianity and allows the performance of Friday prayers?

If you are secure from the trials of desires and doubts, and you are not prevented from performing the rituals of your religion in schools where you find no other alternative, then it is not obligatory for you to leave them. This is on condition that you remain vigilant and attentive, learn the matters of your religion and adhere to them, and seek out supportive Muslim company.