Can i miss school because of diarrhea?

Depends...: Maybe with a flare of inflammatory bowel disease with systemic manifestations. Ischemic colitis, acute infectious bacterlal enteritis may also qualify. Probably not for irritable bowel syndrome, lactose intolerance, uncomplicated celiac sprue. Severity of pathology, dehydration, worsening symptoms may warrant time off. Check with your school to determine if, & how much time, they'll give you.

Good excuse.: I can't imagine that diarrhea would be manageable during a school day, sitting in a classroom for long periods of time. Sounds like an excusable absence.