It depends on the background facts. An employer is not liable for the criminal actions of its employees--since committing criminal acts is not part of their job description--unless it failed in a duty of care. Assault is a criminal act; the school would not be liable or responsible for it unless the school was unreasonably careless. In this context, that would typically mean having had warning in some way (e.g. previous incidents, or at least overt displays of temper; prior criminal record for assault) that this volunteer would either strike students generally or strike this student in particular (e.g. he had threatened this student or there was "bad blood" between them) and, desptie such warning, having failed to take appropriate protective action. Of course, since the legal system only provides compensation for actual costs, injuries, etc., even if the school would be liable, if the student did not suffer any significant injury, there'd be no point in suing--you'd likely spend far more on the lawsuit then you'd recover.