marty's stay home from school policy is this:

if you can't produce blood or puke, you are going to school. if you just think you're going to puke, it doesn't count and you are going to school. if you puke while at school, then you can come home.

as for the blood, i'm pretty sure you only get to stay home while you're bleeding. once it is cauterized or the hemorrhaging relents, grab your backpack. you'll be met at the door.

i seem to recall the rule used to state that if there wasn't blood, puke, or poop (ed. like involuntary poop), you were going to school. i also recall bella looking a little too reflective about the poop option one day and that is how it got dropped from the rotation. hell, i know grown men who'd crap their pants for a day off work. and i got five bucks that says if bella did gravy her drawers to stay home, she'd be so excited, taking a shower wouldn't even be the first thing on her list of things to do with the free day.