If a student purposefully and willfully spits in your face, you should be able to squirt them back with water. It's behavior-modification & works great on dogs.
My other option is use a poster or something as a shield. I'm not allowed to smack their face - which is my gut reaction.
The kid is lucky I have such self control!!
Seriously, I gotta convince the therapist that a squirt of water should be allowable.