It is way to hard to find a shirt that isn't too short for schools (not come up and show stomach when arms are raised up) either have someone make shirts more appropriate for schools or get rid of that rule!
If boys are so distracted by a flash of stomach then they need to be able to learn to get over it. If it's common they wont always be staring.
Plus, it takes a lot of hunting to find a nice fitting, pretty shirt that is long enough. That means you have to go to millions of stores (which uses TONS of gas) and do a lot of walking around and that takes a lot of energy that some parents (or guardians) just don't have enough of with picky daughters and schools.