Should the student continue to sit with his friends who look at and talk about girls at school, or should he leave them?

First, you must advise your colleagues and forbid them from evil. If you hope for their responsiveness, continue to advise them, but do not associate with them while they are committing evil. If you despair of them and fear that you will fall into evil with them, then avoid them in pursuit of safety for your faith, and seek out righteous companions.