My close friend has a son with Aspergers - uber bright, finds it difficult to make and keep friends, often mucks about in class as finds concentration difficult and also if work is too easy he doesn't even want to attempt it (so mucks around, inadvertently disrupting class instead).
During his diagnoses, my friend was told the following (apologies if this sounds crude at any point!): . Everyone is on the 'spectrum'. When being on the spectrum begins to effect every day life i.e. the balance is beginning to tip from being able to cope to not being able to cope, then you need some sort of intervention to put in place coping strategies.
Perhaps your son is beginning to tip the balance and needs some intervention? Not suggesting a formal diagnoses as such, but perhaps a chat with your school senco and putting together some sort of IEP or strategies to enable your DS to cope with whatever he is finding difficult in everyday life? Likelihood is, he's not mucking around because he's a badly behaved child, he's doing it because he's finding everyday classroom life tricky.
Sorry if this sounds all waffly - i'm no expert at all, just have watched my close friend go through the system with her DS and it has changed me from being a judgmental parent ("oh that child is just naughty") to understanding that, more often than not, kids disrupt for a reason. Identify that reason, and then you are better placed to help your kind, caring, intelligent son.