Try getting him to say traffic light colours to indicate his level of anger/frustration, he can use them as it builds up from midly irritated to out of control and also as he calms down. Praise him for using them and ignore the tantrums. Next time your very frustrated with something/one think how difficult it can be to put the feeling into words, a colour is much easier!
Also if he's very clever but angry, it might be worth looking at an emontional cause, perhaps someone is saying things to him that he shouldn't hear, either intentionally or unintentionally, some grown ups seem to think children can't hear there conversations.
I wouldn't worry about the freinds thing, they fall out and make up all the time at this age.
Bella, I just found this website with your post about your 6 year old son. We have been dealing with a similar situation with our 5 year old son since July 2008. He is very intelligent for his age, but we have had a lashing out issue at daycare/school on and off since July 2008. We thought we found the help we needed when he was diagnosed with Sensory Processing Disorder and went through Occupational Therapy. But the issues have continued. Our son will be seeing a Psychiatrist for the first to see if he might be able to help us. Our son hits, scratches, bites, kicks, yells, etc at the teachers and helpers at Daycare/school. (The funny part is he never does it to another child or to us his parents.) We have tried several ideas at home and Daycare/school to no true results. I think I am beyond frustration....
You are not alone in your frustration with trying to find ways to help your son and cope.
Have you received any helpful information since posting your message?
I will gladly share any information that I discover.