What should one do who discovers that members of his university group bribed the professor with a bottle of wine to get grades, and he contributed money without knowing about it, and how does he repent?

You do not have to retake the semester, because the bribe paid was not related to your success, and you thought that what you paid was for printing expenses. You must advise the group of the حرمة (prohibition) of their action, and it is حرم (forbidden) for you to participate with them in studying unless they repent. You must also advise the teacher to abandon what he did, and if he does not respond, then the university administration should be informed about him.