One very positive thing: your son is *talking* to you about his troubles. You must be doing a good job as his dad !
You don't say who you have contacted at the school - but if you haven't done so already, I reckon the Head of Year 2 would be the one to arrange to speak to. Hopefully he or she will have enough experience to know what action to take and not take. Personally, I would be rather worried at the idea of a technology class being left unattended and I would want the school to know that this had happened.
In most schools (certainly the one I taught at), most bullying happens before and after school, at break and lunchtime. If your son fills up his lunchtimes with school clubs and travels home with a friend, he can avoid a lot of contact with these pests.
As well as your talking to the Head of Year - is there a teacher (of whatever subject) that your son gets on with particularly well and can talk to ? If he's lucky, there'll be at least one.

- jeanie