You have to know your kids. I have a child who has something wrong with him EVERYDAY. He has more excused absences than I've had my entire school life. I usually tell them to tell their parents when they get home. My favorite is when they come in school complaining. I ask them if their parents knew when they left home. When they say yes, I tell them their parents think it's okay for them to be in school.
You'll have the occasional good kid who never complains, I let them call home.
We don't have a school nurse, we send them to a clinic, they check out of school, and come back. I try not to send them, b/c it's just like they're checking out.