many things that don't make sense

1) Why was the mother in such a cheerful mood when her daughter had just died? And why is she married to an abusive alcoholic?
She was clearly not all there in the head.

2) How many sports teams are currently competing at high schools the day before graduation? Track, gymnastics, and that kid in the football gear awkwardly passing the ball to himself in the woods.
Apparently they just didn't want to let go of their sports.

3) Why is George Michaels the coach of gymnastics, track, and the shop teacher?
Understaffed school?

4) Why is it sometimes a bright sunny day, then mysteriously night, and then day again?
Bad lighting

5) Why did Anne come home to watch others graduate when her sister was clearly not going to participate?
So that we would suspect she's doing the killings

6) Why was Kevin 40 years old in high school? And why was he such a weapons enthusiast (the weapons mantled on his wall)?
Bad casting. And I guess he was just a weapons enthusiast.

7) Why did Anne hitch-hike home with some creepy trucker (seeing as in the end she caught a cab to the airport)?
She wanted to save money.

8) Since when do seniors dress in their full graduation cap and gowns for rehearsal?
Maybe some schools do this

9) Who shaves their legs in the high school locker room's sink?
Maybe she was on her way to a date

10) What happened to Laura's corpses eyes?
The grandma ripped them out because Laura wouldn't stop staring at her

"Cinema is everything to me. I live and breathe films -- I even eat them!"
- Lucio Fulci

5) She came home to accept some award for her sister. There was a cash component too, and she told the step dad not to worry, he could have the money.