I need help dnt know what to do autistic son

I have a 10 yr old foster son

Had him since he was 7 weeks 

He's in a school for his needs and has some amazing days but others are bad

This Friday he was swearing and he doesn't  usually

But he best his teacher punch after punch , head butting shoving

He isn't violent at home odd slammed door but after Friday I was embarrassed ashamed of him

He has routine good sleep good food  limited TV and internet times 

He had rewards and fuss for good days and consequences for bad days but hed never been that bad the odd shove to another child but this is another lesson do and I need help as I don't want to medicate him