As an FYI, there are different kinds of conjunctivitis - three, specifically.
Allergic conjunctivitis is caused by allergies such as hay fever. This usually presents with profuse, watery discharge and other allergy symptoms.
Viral conjunctivitis is very common with viral infections such as colds. This presents with thin yellow discharge and the eyes look a little bit pink.
Bacterial conjunctivitis, also known as "pinkeye", is the one you have to be careful with and the one that gets students sent home. This one is extremely contagious. The eyes are profusely itchy, look very red, and there is a lot of thick, yellow or green discharge. Often the child will report that their eyes were stuck together when they woke up.
Allergic conjunctivitis is not contagious and is treated with allergy medicine. While viral conjunctivitis is technically contagious, it is not nearly as contagious as bacterial and does not require specific treatment. Bacterial conjunctivitis needs to be treated with antibiotic drops and the child should be kept home.