There are many communicable illness that are common among the school age population. Many of these require exclusion of the child until the child is no longer contagious. If you are unsure about the communicability of your child please contact the health office prior to bringing your child to school.
One very common communicable disease is lice. Although it is an annoyance, lice do not cause disease or harm to children. Please see this link to the American Academy of Pediatrics position on lice treatment.
Lice link to AAP paper: