I just realized that my best friend have been coping me in the last tests, should i accuse her?

So in last school tests i've seen she always stares at my paper (she always sits next to me, she always have bad grades) so today we were in a discussion and at the end before she gets in her car to go she says ''oh! and don't be late tomorrow, i wanna copy you the physics test, just like i did with the science one we did last hour, bye!!'' and its not fair, i mean i get good grades but i worked for them, she doesn't, and help me please, i don't know what to do! in one way, if i keep whit my mouth shut up she'll be abusing more and more and she'll get grades she doesn't deserve, in other way if i say so she'll dump me and put everyone against me (everyone likes her but i don't have much friends) so i was thinking to go with another friend in the recess to talk to the teacher about it, but i don't know, it wouldn't be the first time she betrays me, so it would be a revenge, but i don't know, will the teacher believe me ? help please D: sorry for my spelling this is not my main language.