What should I do in the Fajr prayer if I had a wet dream and could not perform ghusl before going to school? And is it obligatory to perform ghusl before going to school, or is it permissible to delay it until after returning?

One must perform ghusl (ritual bath) after major ritual impurity (janabah) and pray on time. It is not permissible to delay ghusl and prayer until returning from school, as going to school is not an excuse to forgo ghusl or delay prayer.