Had a similar dilema with Dougie starting secondary school.
I spoke on the phone to Dougie's form teacher and sports teacher about ITP and provided them with some info from the web. We also filled in a sheet which the school provided asking about symptoms, what to do in an emergency, and restrictions which they keep in the school office where they do first aid. The teachers work on the assumption that there isn't a problem unless I specifically email to tell them that there is.
We encouraged Dougie to tell his new classmates about ITP because its important that they understand he has an issue if he were to have an accident - older children are less supervised. I don't know how you could do this with a smaller child though - maybe have the teacher talk to Devyns class about it?
I think its important not to panic the school too much, as in most cases and most of the time, ITP shouldn't be a big deal. You don't want them wrapping him up in cotton wool!!!
(or maybe you do?). Hows Devyn doing anyway - still in the heady heights???
Ali
P.S. Julie, I think the Medical ID bracelet idea is a fantastic way of letting teachers know what the situation is - I email, and I never know whether or not the right people have received my email. I might copy you!