X3 Flashback
This sounds like it could be a possibility for the flashback in X3 of when Xavier first met Jean as a child:
Jean’s power first manifested at the age of ten when she unwittingly read the mind of her best friend Annie Richardson, who was dying after being hit by a car. Jean felt the other mind slip away to death and nearly died with her.
For three years, her parents asked many specialists to get Jean out of her catatonic state but only one, Charles Xavier, could help. He discovered that the girl was a mutant and not able to cope with her telepathy yet. So, he decided to mentally block her access to this ability, allowing it to evolve at a natural pace. Even though Jean stayed at her parent‘s home, she had many training sessions with Xavier, which technically makes her his first student (but not his first X-Man) [Bizarre Adventures #27, Classic X-Men #42, Uncanny X-Men #308]