There are too many sensory triggers at school. To much or not enough sensory input. He could be over stimulated. May I recommend a book: The Out-of-Sync Child, Recognizing and coping with Sensory Integration Dysfunction. by Carol Stock Kranowitz, M.A. My copy is dog eared from use. I almost think they should call Autism sensory disfunction. "Just my opinion" No negative comments please.
My guess is overstimulation and the constant demands at school. I work in a school and I want to have a meltdown sometimes. And I agree with Forensicmommy - maybe something is happening at school that he doesn’t know how to talk about or deal with. It’s worth keeping in mind. As my son got older (he’s now 14) he was able to tell me that sometimes another kid would do something to make him mad (as kids do to each other) but he was the one who got in “trouble” for his behavior. He told me that his “behaviors” were the only way he knew how to respond. I praise God that my son was able to verbalize all that. It really helped me understand and empathize with him.
Can someone be hurting him at school? A substitute teacher in my son's contained class terrorized my son and he did not speak for 2 weeks. He literally stopped talking. I figured it out and was glad when my son's teacher came back from maternity leave. What a mean woman the sub was. We are always telling my son to be quiet and stop repeating. However when he does not speak, that is when we know something is wrong so as long as he is in his world talking, he is ok. I hope you do find out as it could be any number of things.
Functional behavior assessment is the best answer for this. Try getting an evaluation for fba which let’s u know the trigger for the meltdown and typically the behavior before and after the meltdown. Is your kid in special education?? U can try for fba in ur kid IEP!