Once a child's mucus turns green, it automatically becomes “radioactive” in the eyes of a day-care provider. However, this change in color and consistency of mucus is often a sign of recovery. The amount of mucus decreases as a child recovers from a cold or flu. The reduction in the volume of mucus in effect thickens it. The green color comes from the dead immune cells that fought the infection.

In general, children suffering from minor colds do not need to be kept home from school. The exception for this rule is during the winter flu season. Unlike the viruses that cause the common cold, the flu virus is extremely contagious. It is easily communicable by air, and if you believe your child has the flu, it is a good idea to keep him home until he recovers. Nevertheless, most children with the flu become contagious a day prior to becoming ill, so school outbreaks of the flu are virtually inevitable.

It does make sense to teach your child to cover her mouth when she coughs or sneezes. This is a simple control measure, but it can make quite an impact. It's also easy enough to learn, even for children less than ten years old. Just remind your child to wash her hands after coughing or sneezing into them. Otherwise, the hands become the perfect vehicle for transporting germs to the next victim.

Infection control in the school setting can be tricky, but the risk of infecting other students in the class appears to be a small price to pay for the gain in valuable educational opportunity. Certainly green mucus is not a good reason by itself to keep away from other students. Children suffering from chronic allergy miss countless number of days of school due to their constant nasal discharge.