Magneto and Pyro would be class 4.
Xavier: "Jean is the only class 5 mutant I've ever encountered"
Obviously Magneto isn't class 5 then.
Callisto: "There are 87 mutants in here, and none of them are above a class 3, except for you 2"
Well, that would make Magneto a class 4, Pyro is above a class 3, but no way in hell is he more powerful than Magneto, so that would make him a class 4 as well.
The others, I don't know, but you could assume that Arclight, Quill, and Psylocke were class 3 at best.