How do you determine too sick for school?

TinyPinkBug

This always seemed like an obvious thing to me, but here I am trying to figure it out.  Ty woke up with a mild cough the last two mornings, but it wasn't bad and he was fine after being up for about an hour or so.  Then, last night he got out of the shower before bed and was suddenly all stuffy.  I put a vaporizer in his room, some Vicks, etc.  He slept great but woke up with a terrible barking cough.  I took his temp and it was 100.0 in his ear.  I gave him some plain old Tylenol, and a half hour later he is totally find again.  No cough, no stuffy, no temp, just grumpy but there is no rhyme or reason for that when it comes to Ty.  I was certain he was staying home and maybe even going to the dr when he got up, but now he is fine.  We have been spending time outside, 3 hrs last night.  Could it possibly be allergies?  Keep him home?  Send him in?