Is a student's refusal to help her classmates with their studies, or her refusal to explain lessons to them, or to give them good exercises, considered envy (hasad), and what is the ruling on that?

Not helping students, when one is capable of doing so, unless it is something required of the student himself and in which help is not permissible, goes against Islamic brotherhood, which commands that one should love for his brother what he loves for himself. The Prophet, peace and blessings be upon him, said: "None of you truly believes until he loves for his brother what he loves for himself." Allah also condemned in His Book those who withhold kindness from people, saying: "And withhold [even] the small necessities." Therefore, one should not refrain from helping female students in explaining lessons and similar matters that do not constitute cheating. As for not helping, it is not considered envy, for envy is wishing for the disappearance of an existing blessing.