Does anyone know how long your contagious after you start on antibiotics??

Depends on your disease and what antibiotic you're taking.

i'm taking a zpak for sinus infections/bronchitis.

The pharmacist/doctor call would be the sure bet, but you could check webmd.com for a quick (possibly inaccurate) timeframe. I'm sure your doctor could tell you really quickly, though.

yeah i'll just have to call. its no biggie, we just have a lot of newborns in our hall and i'm not going to go the the meeting if i'm contagious.

Maybe you'd want to sit in the second school, then, and just keep 'em away from your phlegm. :)