Marty and Jennifer worrying about the principal catching them.
The scene where Strickland chews out Marty sets things up that happen later, but I always thought it didn't make sense that were worried about him giving Marty a tardy.
Wouldn't he have been caught by the teacher when he got to class late? Teachers always took attendance at the start of class when I was in school.
It would've made more sense if they didn't want him to catch them because he's a dick and they didn't want to deal with him. I guess you could argue that Marty's homeroom/first period teacher might let it slide if they liked Marty, but I don't remember a teacher ever doing that.