What should I do when I face mockery and ridicule when advising my friends at school, and should I continue to advise them while enduring that? And is it obligatory to advise someone I don't know, especially if I fear harm from doing so?

The questioner should continue enjoining good and forbidding evil, and be certain of what he enjoins and forbids. He should not care about the ridicule of mockers. He should be gentle with his colleagues and explain to them the sayings of scholars calmly and kindly, mention convincing evidence to them, and show them beneficial fatwas. As for forbidding evils in the street, if he fears harm, he is not obligated to forbid it. The same applies if evils become numerous and it becomes difficult to forbid them.