Damn computer ate my edit!
I echo what frizzle has suggested. I wouldn't push any further with the punishment under the circumstances you've described. I think Zombie Mum raises some great points as well -- undesirable behavior should be met with natural consequences, rather than something totally unrelated (taking canteen money to "punish" for taking Lego -- that's confusing!)
Social stories can be very valuable in setting expectations (for use in new situations) or reminding childen of what should be done as a matter of routine (spelling out what things need to happen when arriving/unpacking at school). If you pop over to the SNs board, I'm sure several of us there could go into greater detail about things that have worked well with school aged kids.
In terms of classroom behavior management, what has worked very well for our daughter (this is under the direction of our psychologist, who is also a board certified behavioral analyst) is developing a simple sheet of performance expectations at school (3 or 4 core things). She has had a hand in the drafting to enforce the notion of ownership.
If she demonstrates good behavior (asking my teacher for help when I have a question or am upset / using my brain to try new things / listening to my teacher / etc.), then she gets a small token for that period. If she collects X tokers/stickers, then she earns 10 minutes toward the end of the day to do creative writing, which is her passion.
A key success factor is that the rewards for good behavior at school be tied to school and that they be somewhat immediate (rather than "If you are good all week, I'll take you to the Lego museum.")
When kids have obsessions, it actually gives you an opportunity to leverage their "currency." I imagine with a little thoughtful consideration, you & your son's teacher could come up with something that helps reinforce general "good citizen" rules and also helps motivate your son.
Are you working with a psychologist or behavioral specialist at present? If not, I would strongly suggest that you get a pro on your side who can help you anticipate challenging episodes and give you some guidance on how to address them.
Edited by baddmammajamma, 05 February 2013 - 09:55 AM.