What should a 15-year-old do who studies in a school where pornography is widespread and where he is exposed to harm and harassment, knowing that he has repented from practicing masturbation?

Praise be to Allah for your repentance from masturbation. You should try to transfer to a better school if possible, and you must stay away from bad companions and seek help from those who can deter them, such as a teacher, a parent, or good friends. Try to find a righteous companion with whom you can cooperate in this corrupt environment.