This answer does not establish an attorney client relationship, nor does it constitute specific legal advice. The answer to your question may depend on several facts which are not present in your question.
If your daughter is in public school and this is the only offense she has committed it seems highly unlikely that this incident would be sufficient to warrant expulsion. Each school and/or district publishes a disciplinary policy manual and distributes them to students and parents. In this manual most districts have a matirx that describes the acceptable punishment for violations of the disciplinary code. If you cannot locate the copy provided to your daughter, the school/district should provide you with another copy.
If expulsion is an acceptable punishment, there are district, and often school, level procedures for appealling the expulsion decision. These procedures may be deceibed in the disciplinary manual. If not, you can contact the school or district to find out how to appeal such a decision.
In your case, I would forst advise contacting the school principal and requesting a meeting with the dean and the teacher present to discuss the situation, review the evidence against your daughter, and discuss the appropriatenes of the disciplinary actin the school intends to take. If further action is required, I would recommend contacting an attorney who has experience in education law and in dealing with schools and school districts.