What can be "YL" classes?

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May 09th 2012 19:21
A teacher is on detached duty to cover another teacher's classes at a branch teaching center at the moment.

He said that he is having a real hard time keeping his temper teaching 50% YL classes in a job that was supposed to be exclusively adult courses.

In this context what is "YL"?
I looked up a dictionary and it said it could mean "radical". Is it right?