confused about all these vaccinations posts..

Correct me if I'm wrong... but I thought in order for a child to be in daycare, school, etc. they had to be vaccinated? Isn't that why they want records of all their shots and make you get certain ones before they are enrolled? I'm just confused on how an un vaccinated child is able to attend school. I was under the impression that it was not an option. Like I said, I could be wrong. I'm just curious.