I talk to them for a moment to decide the severity of the issue and then tell them they have to go back to class. If there is no fever, no puking, no diarrhea they stay at school. Now if I've noticed they are acting droopy or lethargic I might go pick them up and take them straight to the doc's.
I will often ask the school why they called after talking to the child. They say, your child said they were sick. I ask them if they were actually doing anything that was cause to send them home, they'll often say no, that the child just didn't feel good. I tell them the kid is fine and they need to go back to class. The calls have stopped a bit since the kids figured out that it is doing no good for them to call me.
When I was a nanny for a family with 7 children plus a spare, he was the mom's bosses son, I still had 2 home all day, 1 1/2 half a day, and the rest were in school all day. Now, I was a funny nanny sometimes because I'd pack the little guys up and we'd go to the park and then out to lunch, we might order pizza or Chinese for lunch instead of cooking, maybe the mall, just get out of the house for the day and have fun. I didn't really need the money I was making, I just enjoyed working so I was always spending money on the kids. I was their surrogate mom after all. That was in my job description, seriously.
The kids started seeing how much fun we had and started getting where they would want to stay home too. Pretending to be sick was pretty much "the way" for that to happen.
One day I got to work and I had my regular 2 guys and then the next two. The half day kindergartner and the 1st grader both stayed home. They weren't really sick of course but wanted to stay home and have fun, eat junk food, and play.
So, I made them stay in bed all day and I fussed over them. I tucked them in, I kept feeling their foreheads, made sure they had lights out so they could nap, when they asked if they could have Egg Drop Soup from their favorite place I told them no, they could only have water and broth, since they were feeling sick to their stomach the soup would be bad for them. Poor kids, they really were counting on getting to go do a lot of fun stuff during that day. They didn't get that reward. They didn't ever stay home from school again if they truly weren't sick.