What should i do if my daughter is getting teased because of the way she looks and acts?

my daughter, maggie, was born with a congenital disorder which makes her look and act differently from other people. she goes to a special school because she also has learning difficulties. she went to play outside while i was making lunch. maggie met two children about her age while she was out. their names are linda and peter. they are twins and they were playing with a ball. when they saw maggie, they laughed at her. they teased her and made fun of the way she looked and acted. they also called her a stupid retard. the behaviour of linda and peter really hurt maggie. i know how sensitive my daughter is. maggie angrily snatched away their ball and took it away. what should we do? what is the best way to settle this incident?