catok,
How well do you understand autism? Sorry to ask but as parents we need to be our kids best advocate, especially in schools where they do not understand the issues.
Many are sensitive to noise and this can be very alarming. The same with when there is a lot happening around them.
Self exiting is not being naughty, it is an important part in self manangement of his aspergers.
Children with Aspergers have to work very hard to maintain control at mainstream schools. It is not always possible for them to do this and it is when they feel out of control and unable to cope that much of the disruptive behaviour kicks in.
Your son is finding a safe place where he can calm down and regain control. It is quite a mature approach and if understood appreciated.
This is a well established technique in helping children with Aspergers to cope within mainstream. What the school should be doing is finding a way to enable him to let the teacher know he needs a time out and providing a place where he can go and calm down.
The HT shouting at him and telling him he is naughty is just making things worse and would be stressing him out much more. For one thing he is not being allowed to calm down. It is also undermining his confidence and besides which he is NOT being naughty.
The school has external agencies it can go to to get advice and help on how to help your son cope much better in their school. But they do need to ask for the help.
As hard as it is to understand many schools and teachers do not understand autism or the range of effects it can have on individuals. If his teacher is an NQT the chances are they will have no idea about it.
If the school is reluctant to do anything there are things you can do. Contact the education psychologist and explain what is going on and ask for help. Your local authority should have a designated person responsibility for child safety, this is someone else you can approach.
Someone you may not think about is your local councillor. Some are much better then others, however they are in the position to talk to people you would probably not have access to and bring pressure onto the school from above. I have successfully used this route.
However your understanding the issues and being able to explain them are going to be the key in being able to advocate for your son.
If your sons diagnosis is new, or you are not sure/confident in explaining it all give ta.forumup.org/ to the HT, class teacher, CA, LSA and SENCO.
It is a support forum for teaching assistants and includes a range of support staff and a parent with experience and understanding of autism. They are very supportive and non judgemental when people are genuinely looking for support and help. It does have a folder specifically for autism/aspergers.
Hopefully at least one of them will bother to at least check it out because by the sound of it they are doing everything wrong!