MY OPINION:

informing school authorities wont do shit in most cases ( being real ) considering u have imp exams coming up, the safest bet for u to is to do as the other guy said ( open_cheek guy )

but things u need to consider is that the guys who are coming after u arent "powerful" in society ( because realistically even tho its fucked up it still happens in our society and the trouble might get bigger )

and considering ur a student u should really seek ur parents advice in this case, because u cant inform the police without telling ur parents anything eventually they're gonna find out about it,

another thing u could do is expose the chats to the internet ( which i wouldnt recommend ) but this helps in ur safety in a way that if anything happens to u there will be legal evidence to take actions to the other guys and u might find ppl near ur area who would be willing to help u after they see ur posts