Thought is faster than moving ice. Professor X could "freeze" anyone before they could even move.
I can understand Magneto being under Iceman, because Magneto, besides flying and some other abilities, is highly dependent on his environment for his power to be truly effective, whereas Iceman generates his own power. But there's no way I can put him above Xavier; he may not be able to kill every single person on the planet without Cerebro, but he can readily do it to everyone in his vicinity, no matter how powerful they are.