There's no rule.
3 seconds sound clip from the Donnie Darko movie soundboard.
You can hear this line at 00:30:05 in the Blu-ray version of the movie.
Quote context
[...]
- 'The Last Unicorn' by Samantha Darko. 'There was once a unicorn named Ariel...'
- It's not wrinkled, Sam. Just flatten it in a book or something.
- It's almost 7:45. The bus should have been here, like, 20 minutes ago.
- Maybe Martha Move Funny went nuts and hijacked the bus.
- Hey, you know, there's, like, this rule. At 7:45, we get to go home.
- Yeah, if you're still here and the bus comes, we're all gonna get in trouble.
- Hey, Porky Pig, I hope you get molested.
[...]