You are your son's protector... and you are trusting the staff of your district with his care. If they are not living up to what you expect of them - tell them. After you talk with the driver/teacher, the next step it their bosses, and if need be work your way up to the superintendent of the school district.
All schools have rules - usually one is hand & feet to yourself... it sounds as if that rule is not being followed. And I'm sorry, but if my child was strangled during any part of the dayd - I would be really PO'ed... and if I was only informed of what happened by my child, I would be even madder! Sorry, but I'm sorry is not all that child that strangled your boy should get as punishment!!!
State what you expect of the school & why you are upset... just try your best to stay respectful & don't yell or use bad language. Also, sometimes it is better to send/e-mail in letters then to talk to them... letters are proof you have tried to address the stiuation - talking has no proof - it is just your word vs. theirs & most people do lie when they are in-trouble for something.
Good luck - I hope they situation is handled befor your son dislikes school!