What should one do when they advise others and are mocked, especially when they are 16 years old and advise their schoolmates not to cheat?

You have done well in advising your colleagues against the impermissibility of cheating. You should continue to do so, choosing an appropriate approach, and do not pay attention to their mockery, for that is natural on the path of da‘wah (inviting to Islam). Rather, persevere in your approach and invite some righteous individuals to help you. We advise you to be patient, for: “And enjoin what is right and forbid what is wrong and be patient over what befalls you. Indeed, that is of the matters [requiring] determination.” Indeed, the outcome of patience is relief.