What is the ruling on forging an exit pass to leave school, with the possibility of having an excuse for it, and is it obligatory to inform the administration about this act?

Falsification is not permissible Islamically except in very narrow circumstances, such as in the case of injustice. As for informing the administration about your colleague who falsified, if his situation is bearable, then do not report him to the administration so that he is not wronged. Rather, suffice with advising him and warning him of the consequences of falsification and cheating.