I work in a school office, and please forgive me for ranting a little here....
Saying to children "I know you don't feel 100% but go to school and then if you get worse/don't feel better, go to the office and they will phone me and I'll come to get you".
Believe me, the child will report to the office about 9.15 am and then at 30 minute intervals until we phone you to come and collect your son and daughter.
By all means, tell the teacher or the office that you have dosed up your little one to the eyeballs with Calpol and that when it wears off, you can collect but don't let your child hear!!