Ok then... I can explain it.
Let's open up my NPC making guide, High School Years pack... Ah, here!
You need to equip this trait to the teens: trait_HighSchool_Active_Class1Student
Then the teen has a chance of appearing as an NPC in the active high school.
Otherwise existing teens are pulled and put into that role. I hope that the game assigns that hidden trait when your sim attends the active high school, but I'm not sure.
So yeah, the main reason is that they don't have the required hidden trait. Cheat to equip this trait is here!