The school should have an anti-bullying program/curriculum to handle the nonsense with the other girl. No breaking crayons in the face, and that sort of thing.
I'm not sure what sort of drugs you are referring to, but if the other mom is using illegal (or prescription) drugs and has erratic behavior, the school will notice it. If she took too many cold pills and had an adverse reaction, she may not remember it or may feel badly about it and will be on her best behavior in the public environment of the school. She may even be volunteering there to sort of redeem herself in everyone's eyes.
This is a good opportunity to teach your daughter about dealing with difficult and irrational people, about being respectful and yet making better choices. The problem girl will not have any friends if she continues to act like this, and your daughter's patience will win out. It can be tough on a new kid in school, but let your daughter know that sometimes another kid can feel insecure or threatened by a newcomer, not the other way around.
I would not deal with a lawyer or start threatening the school. They are well aware of their liability. It would make much more sense to consider the teacher and the principal to be part of your team to ensure your daughter's education and adjustment, rather than "put them on notice" and be adversarial from the get-go. That will put you in the role of "trouble-maker" and that's not where you want to be. If you and your daughter are the reasonable and cooperative ones, it will be much better for you. It sounds like you are already on that road - your daughter was willing to cooperate but the other girl was not. I don't think this was lost on the principal. And, as I said, if the other mom is suddenly showing up to be seen in a good light, that's not going to be lost on experienced educators who have seen parents try to suck up and get on someone's good side.
The school should have very strict rules about teachers socializing with parents anyway, so I'm not sure why you assume that the teacher must have been friends with the other mom. The teacher is absolutely prohibited from discussing any child with anyone outside of the classroom. I would ASSUME that the teacher is doing things right rather than assigning blame for a breach in confidentiality. I'm not sure why you are blaming the teacher for the other girl's behavior - I understand that it sounds like she "must have" said something, but it could also be that the mother spoke to her daughter and they are seeing you and your daughter as the causes of the problem, or at least are trying to cover their butts on this one.
So, I think you will damage your own standing if you are adversarial or accusatory toward the other teacher and the school. Tell you daughter to be respectful, fun-loving and high class, and she will attract other nice kids. Take the approach that the principal and the teacher will do the right thing - if you need to meet with them, tell them factually what you have observed (without value judgments on the other mother/kid) and realize that they cannot discuss those people with you, just record their behavior. Ask for their cooperation and develop a team approach, asking how you can participate helpfully and positively. Don't threaten legal action - they absolutely know about this already and you don't need to throw it in their faces. You don't want to become "the problem" - you want to be part of the solution.
Good luck.
