I have never sent my child to school sick, I am sorry if you caught something from one of the children.
(Warning, the next is a bit of a rant)
I just want to say however that I have had a bad experience with my son's school (he is in the state Pre-K program that is contracted out to a private preschool) trying to tell me he was sick when he was not. I do understand that some parents sent their kids to school when sick, but please never make the mistake of telling a parent you know more about their child's health than they do. A few weeks ago, I dropped my son off at school and 20 minutes later I got a call at work saying I needed to pick him up because he had pink-eye. Pink-eye that he showed no signs of 20 minutes earlier. I let the director of the school know his allergies were bad that day, and he had probably rubbed his eye because it was itchy. She said I had to pick him up. I got to the school, and was told his eye was crusty and red. In reality, he had a speck of crust in the corner of his eye (his allergies cause blocked tear ducts, as do mine) and his eye was maybe a SLIGHT shade of pink right in the corner. I wiped out the offending crust speck, and 30 seconds later his eye was white as snow. I was told he could not come back until he had a doctors note confirming he did not have pinkeye, or until he was on antibiotic eye drops for 24 hours. I then had to drive an hour to my MIL's house so she could watch him for the day, an hour to pick him up, and then take him to the after hours clinic (and pay for the appointment) so they could ask what the heck we were bringing him in for.
The next day I took him to school with the note that he was OK to return to school, he did not have pink eye. They still tried to make me take him home because the note did not have a date specified on which he could return!

The whole episode actually made me seriously consider sending him to another school.