At my child's school they sent home information about a contagious bacterial disease called impetigo. It needs to be treated right away if it is, you don't want to infect other children.
http://kidshealth.org/parent/infections/bacterial_viral/i...
Impetigo, a contagious skin infection that usually produces blisters or sores on the face, neck, hands, and diaper area is one of the most common skin infections among kids.
It is generally caused by one of two bacteria: staphylococcus aureus or group A streptococcus. Impetigo usually affects preschool and school-age children. A child may be more likely to develop impetigo if the skin has already been irritated by other skin problems, such as eczema, poison ivy, and insect bites.