There are many contagious diseases that are common in the pediatric population and thus in the school setting, the most common include:

Common Cold
Conjuctivitis (pink eye)
Fifth Disease
Hand Foot & Mouth Disease
Noro Virus ( i.e. stomach "flu")
Staph/MRSA
Strep Throat

These diseases are so common that families are not notified of the conditions circulating in the school unless the incidence is more severe or more frequent than routinely observed.

Though  much less frequent than before routine immunization, Some vaccine preventable diseases (VPD) occur in the school setting:

Influenza
Pertussis
Varicella 

Other less common  communicable diseases/conditions may occur which are more severe in nature.  These diseases are routinely monitored by the local health department (LHD) and may require population based interventions.