One of the necessary ingredients to any lawsuit is that there must be damages. These can be real damages as in money or property, or sometimes for intangible things like emotional distress, etc. Anytime that a lawsuit is started over the 'intangible damages' it is MUCH harder to prove, since there aren't any pictures or evidence that you can show the court (or jury). It becomes a matter of who's 'expert witness' is more credible.
As you can imagine, due to the nature of these types of cases, they are expensive and hard to win. That means that, even if you do have a case against someone (and I am not sure that you do), you will need to have a wheelbarrow full of money to even get a case started. I know that doesn't sound fair... but that is the reality of the world.
So, with that in consideration, lets look at what possible case you might have.....
Teacher: Certainly you might have a case against her, if you can prove that her actions were negligent or malicious and if there is any chance of recovering anything from her.
School Board: Depending on the circumstances, if the board allowed the teacher back on campus and 'aided' in her being able to re-approach your daughter, there could be a claim made against them. But, they will be able to finance a defense much better than your chance of an offense.
Bottom line...
Depending on the full circumstances of this case, you might consider at least talking with a local attorney. There are some out there who might be willing to help due to the nature of the complaint. At least they could write the board a letter and possibly prevent this sub-teacher from being allowed to harm others. And you might consider contacting your state or local Education folks to see if you can get that teachers 'license' suspended.
Best of luck to you....