If something happens, you will not be held for punitive damage unless your daughter's conduct is gross negligent, reckless or intentional. Punitive damage is something recoverable in ordinary negligence. How much the punitive damage totally depends on your financial situation. If you are terribly worried, get an umbrella insurance.
Since you have sole custody, and your daughter is a minor, I suggest you get her back immediately. California schools are ranked 47th in the nation. Don't you think you can find a better school in your state?