That comes after confirmation that an 8th grader has the disease, also known as pertussis.
The Montgomery County Health Department says the student did not need hospitalization.
Everyone who has been in close contact is taking preventive antibiotics.
The student had been vaccinated as a toddler, but that immunity can fade over time.
The number of cases of pertussis is rising here and around the country. Preliminary data shows Montgomery County had 141 cases last year, compared to just 31 in 2007.
Other counties in southeastern Pennsylvania reported these 2010 numbers to the state:
Philadelphia - 143
Delaware Co. - 64
Chester Co. - 63
Bucks Co. - 152