When you say do they definite her,do you mean should you let her actions then affect your view of her now?
Depends.
Assuming this was a one off, not part of a pattern that continued into adulthood:
Generally speaking 15 is a point when empathy is not fully formed and decision making is immature also there is a lot of variance in parental and social influence. Someone could have been raised thinking that this is an acceptable way to deal with conflict, but have since grown up to understand that it is wildly inappropriate and disproportionate.
15 is above the age of criminal responsibility so I would say the individual is accountable for their actions, therefore I would want to know how they are now rather than discounting something so serious as having no relation to their current character.
How does she talk about these allegations, does she show remorse?
Also what was the outcome for the teacher, was there an investigation and if it found in her favour, has she ever tried to put this right?
If the teacher's reputation and/ or livelihood was destroyed and she, as an adult, doesn't see a problem or finds it funny, then no, I wouldn't want anything to do with her. However if she sees it as a regrettable mistake, an immature and impulsive decision that would never happen now then no, it doesn't define her.
It's a big, vicious fabrication as teenage porkies go, but if she has reflected suitably, come clean, and substantially changed her strategies for dealing with adverse situations then that's not who she is anymore.