Raven Darkholme had been christened in her second year as 'Mystique', due to her uncanny ability to imitate other people, and her knack for appearance-changing spells. She had also managed to steal a recipe for Polyjuice Potion at some point in her school life, and took great delight in telling her classmates off while looking like a teacher.
Every one of Raven's friends had a nickname. Some called her little group the 'X-men', or the 'mutants'. Raven was a little flattered, honestly.
There was her longtime best friend Charles Xavier, a brainiac Ravenclaw with thick dark hair. His biggest fear, much to his friends' amusement, was going bald. Charles was gifted at reading body language, and would often say what you were thinking before you even thought it.
Then there was Logan, or 'Wolverine', a Gryffindor who had stolen some dodgy Polyjuice from Raven in fourth year and had turned temporarily into his nickname. He was fierce, temperemental, and liked sticking knives between his knuckles and pretending they were claws. He was also the Gryffindor Keeper.
Hank 'Beast' McCoy, another Ravenclaw, too smart for his own good and occasionally so blindly ambitious you could mistake him for a Slytherin. He had slight anger issues, not unlike Bruce Banner, a Ravenclaw that everyone liked who tended to stick with his own group (the 'Avengers', if Raven recalled correctly).
Alex 'Havok' Summers, a handsome blond Gryffindor with a great like of blasting spells. There were rumours he had a crush on their friend Armando Munoz, and Raven was inclined to believe them. Havok was also the star Beater for Gryffindor.
The aforementioned Armando 'Darwin' Munoz, a Muggleborn Gryffindor, whose hero was Charles Darwin. Darwin was the Transfiguration teacher's favourite student.
Erik 'Magneto' Lensherr, a Muggleborn Slytherin with patchy self-control. He was gifted at both Charms and Transfiguration, particularly with metals.
Sean 'Banshee' Cassidy, a ginger Hufflepuff who was sometimes mistaken for a Gryffindor, due to his reckless flirtation methods and boldness. He had a very loud, high-pitched scream, as found out after he was knocked off his broom (he was a Hufflepuff Chaser) in a Hufflepuff vs Gryffindor match.
Angel Salvadore, the only one without a nickname. An attractive Slytherin with a pair of wings tattooed on her shoulders and a weakness for a good fireball curse. She was a Slytherin Beater, losing the spot of Seeker narrowly to her teammate and 'Avenger' Natasha.
I'm only really using X-Men First Class mutants, as you can see - with the exception of Wolverine.
X MEN SORT:
GRYFFINDOR - Havok, Darwin, Wolverine
RAVENCLAW - Hank, Charles
HUFFLEPUFF - Banshee
SLYTHERIN - Erik, Angel, Raven