Drillbit Taylor plot holes.

If Filkins is 18, then why is he any of the same classes as the main 3 boys, who are all freshmen? (We see Filkins, Wade, Ryan and Emmit in the same classroom for English and Gym at least once).

When Drillbit impersonated the substitute, how did he know the exact teaching schedule of the original substitute and never get caught because he didn't show up for a class that the substitute was supposed to teach?

Why did nobody in the school ever ask Drillbit for ID to prove that he was the actual substitute?

What happened to the actual substitute who was supposed to take over the classes that Drillbit taught?