ASD and shouting out in school

My son (9 years old) is undiagnosed ASD with sensory issues and maybe PDA. We are waiting for an ASD assessment following a consultant mentioned she thinks he is high functioning so now he is in limbo thinking he has autism and myself and school are assuming he has.

He has always struggled with school (home seems manageable) but this year he has had a bad teacher who is very strict and doesn't seem accepting of SEN. He started shouting out in class, sometimes random words but usually just answers or to his friends. We have all told him he can't do this as it distracts other but still we can't find a way to stop it.

My question is, should I assume my son has no control over this behaviour (as he seems to continue after all the school meetings, upset parents etc) or is he actively choosing to do this for attention or boredom? Does anyone have any experience of this and know of any strategies that have worked? I have googled and read a lot!

Thanks