Administrators at Ridley High School said three faculty members and two students are sick, and there are other probable cases.
A notice also went to students reminding them "Don't share eating utensils or drinking straws" and "Wash hands thoroughly and frequently."
Delaware County Senior Medical Advisor Dr. George Avetian says mumps is spread through saliva or respiratory secretions.
There are a few common symptoms.
"As with many viruses fever, malaise, headache," said Dr. Avetian.
And the telltale sign - swollen salivary glands
"So you will see the glands in the front of ear by jawline puff out," said Dr. Avetian.
The superintendent said the majority of students at Ridley High have been vaccinated, but as we've seen in other mumps outbreaks, like at Temple University last spring, the vaccine isn't perfect.