A nearby district has a lottery system for their GT magnet. They do not have enough space, and that is why they have the lottery. First kids must be identified as GT, then they can apply. It ends up being a school full of the range of GT kids, as there is no preference for HG+. I have heard parents of HG+ kids say that it was a horrible fit for their kid, as they were bored and the school didn't do extra differentiation, but then more recently I've heard that they school has provided more options for HG kids. Depends on the administration, and the kid, I would guess.
I would say a lot depends on how flexible the school is. I would ask if there are different groups for different kids with different abilities. Is subject acceleration allowed? Do they cluster or group together kids with similar abilities? I would be wary if they said "our curriculum is challenging for all our kids." But if your son isn't really getting much at his level now, it may still be an improvement.
Can you ask for contacts so you can ask parents with HG kids how they like the school?
Are there any HG magnets around?
The Davidson Institute Resource library has a good list of questions to ask of any school:
http://www.davidsongifted.org/db/Articles_id_10511.aspx
