You don't mention your son's age, but jewellery policies differ from school to school. Most say that a pupil can wear studs in the ear only and all other items of jewellery must be removed. This has historically been for health and safety reasons, there have been many incidents in schools where damage to pupils ears has occurred from accidents involving jewellery.
I agree young people should be able to express their individuality, but as the sister of a piercer,I know that stretching cannot really return to normal size, would you let him get a tattoo?? This will be in his ear and visible forever. It can also be dangerous, my sister has stretched both her ears, and one has not settled right at all, the skin is tearing and she is a professional having done this for many years. It can happen to anyone.
Self expression at school in my opinion should be limited to hair style, perhaps colour (not bright blue or green though), and not by irreversible body modification. In this instance I am on the side of the school.