I have heard that some schools have a weigh-in policy for their girls, but I don't know which ones. Personally, I think that is terrible and unnecessary. If anything, it's a likely factor in contributing to eating disorders.
I don't know which schools have the heaviest cheerleaders, but it probably would not be those schools which are famous for their cheer teams, and more likely to be an all-girl team than co-ed. Heavy cheerleaders can be very helpful in basing stunts, so as long as they can successfully perform their cheer skills I would put them on my team.