I ask for advice, dear GFF.

No shame.

I was in pretty much the same situation as you, although it's hard to get an accurate feel for you , cause i went to a very small private school, and it sort of gets to the point where you're friends with everyone regardless of what emotions are flying around. but i digress.

In this situation, all i can say is, what have you got to lose? You're (possibly) never gonna see her again after you graduate, so why the hell not just let go and tell her everything. If she says yes, you two enter a relationship and stick together in the coming years. She says no, you go your seperate ways as was originally "the plan" and nothing changes. This is just the situation you have to see as a situation with little to no negative consequences. You have no contact with this girl outside of school, and you're about to finish high-school so there's not much to salvage in this situation.

The only red flag i sense here is the fact that she had 2 boyfriends at once. If i saw that in a girl i'd back off, and stay the hell away. 2 boyfriends at once is often bad news, and she'll either dump you quick for another guy, or worse, she won't dump you, and lead you on while she's seeing some other guy or fooling around behind your back. I don't know this girl though, so I might be making a mistake by saying that.

Jam it back in, in the dark.