i see paola just answered your question but i just typed this and don't want to delete it. :>
emma started being 'evil' in the hellfire club. when she and shaw took over, she started recruiting young mutants so they'd help their cause. after a battle with fitzroy all of them died and she was left in a coma. so the professor took over the massachusetts academy. when she recovered she was again headmistress.
as to powers, she can read minds and project her own to control people. the diamond skin thing (which makes her resistant to injury) is a recent addition.