Hi OP.
My DS was wonderful at primary school. I never received one phone call about his behaviour.
Then he went to secondary school. Within two weeks he had his first fight. His behaviour got worse and worse and worse and I received endless calls from the school.
Eventually, last year, he was permanently excluded. In his case this was the best thing to have happened to him. He has gone from an aggressive, hostile teenage boy who had virtually stopped talking altogether back to his old personality. He is happy and calm and talks almost constantly. He has also been very recently diagnosed with ASD.
It appears, from what the psychologists said, that he could cope with primary school and masked the ASD because he felt safe and secure there. He simply could not cope with secondary school, the noise, the number of students, the moving from class to class, having to remember where he was going and his equipment. The list goes on and on.
His school told me, in writing, that my DS was just badly behaved. He was badly behaved but he was also desperately unhappy and could not cope. He was self harming (cutting) in places I could not see. He now has scars all over his legs and upper arms and pelvis.
It took me over three years to get my son the support that he needs. Please don't write this off as just being the class clown until you have drilled down into what has caused this change in his behaviour. It could be lots of things.