If you're attacked, your response should be based on the self-defense laws in your area. How that response--and the tools you may use to implement it--compares with the school's rules and policies is the tough part. If attacked on the street, your right to self-defense may be clear. By being in the classroom, an otherwise legally justified physical response may be inconsistent with the school/department of education policy. Your best bet is to choose up front what's more important: your safety or your job. Choosing in the midst of an incident won't work because too much fur will be flying.
Stay safe,
Mike