I don't see a problem either. In fact I think it's a great idea and an tempted to suggest the same for my kids teachers. An iPad is not $500 and could be a great addition to the classroom.
I love when the school asks for donations and then divides the money up per teacher. They probably prefer the cash and get a lot more that way.
Full time high school administrator and wife! Full time mom to Gabby (9/04) and Anthony (1/08)