Ninja can only train a jutsu's with certain ranks.
Genin can train: E, D, and C ranked jutsu
Chuunin and special jounin can train: E,D,C,B, and A ranks
Jounin can only train E,D,C,B,A and one S ranked
Sanin can train all ranked jutsu Including X ranked jutsu
In order for say a genin to train an X ranked a sanin would have to teach them. This is to make things realistic another way for genin to learn X ranked is to steal a village scroll such as naruto did to get a B ranked jutsu