Complex sentence: The original, and most notable, group of Hellions were students of Emma Frost and trainees of the Hellfire Club, attending classes at the White Queen's Massachusetts Academy while training in the use of their powers in an underground complex beneath the school.
Split into simpler sentences: The original, and most notable, of the groups of Hellions were students of Emma Frost and trainees of the Hellfire Club. They were attending classes at the White Queen's Massachusetts Academy while training in the use of their powers in an underground complex beneath the school.