This school is not fulfilling their duties. If they accept an autistic child they must provide the guidance and they definitely have to protect Mo. I was on the board of a non-profit that provided job training and socialization for adults with developmental impairment. It was a day program, and it's important to realize that within that population we had several folks who could be harmful to others. One young adult liked to flip wheelchairs especially if they were occupied. She knew she was wrong, but she would take off running faster than any of the caregivers could and flip those chairs. When all efforts to retrain her failed, she was dropped from the program.
The one that really stands out to me was the 18 - 19 year old autistic man who would go into rages when he didn't get his way. He was over 6' tall and weighed close to 300 pounds and he scary. I was in an office next to a room where he was with several others, something made him angry and I was very startled by the volume and fury in his voice. Then he began pounding on the wall between us so aggressively the pictures on the wall shook violently. I asked the staff member what was happening and that's when I realized how bad a situation was for the other clients and staff. The organization tried everything to control his actions, but one afternoon he send a staff member to the hospital and they had to send him home and his Mom had to find another place for him.
I think both the parents and the school are seriously misguided about their responsibilities. There are useful programs for autistic people and your school isn't one of them. I would ask the administrators whose insurance covers your son from violent behaviour while he is in their care - the school or the parents or possibly both. Morally and ethically they cannot ignore their responsibility of care by shoving it off to a 15 year old child whose parents pay tuition. even if they don't charge tuition, it doesn't matter, this other boy is a threat to Mo.
If it is a public school, you still have options. I'll see if I can get in touch with the staff members and see if they have any useful suggestions. Bottom line, autistic boy is snowing his parents. And they have to follow them with the treatment at home if they hope to prepare their son for eventual independence.
I am not unsympathetic to children with developmental impairments, but within those groups you will have nice people, bullies, con artists, and malingerers. They may think this child can't help it, but he's targeting Mo, so now it's up to them to correct this situation. Please don't feel guilty or worry that others think you are intolerant of autism. That's not what's going on, and like I said earlier, our autistic man injured several people during his brief stay at our facility. The school can't just throw up their hands as if they are not responsible.
If you have any questions you think I might help with, please just pm me. I don't have all the answers but I try. Good luck boomer, I hope this gets resolved for Mo's sake.