Sixteen students at a secondary school in Nottinghamshire are suspected of contracting mumps.
Public Health England (PHE) said it is investigating a number of reports at Toot Hill School in Bingham.
In a statement head teacher Sandy Paley said they are "working closely" with PHE and advised all parents and staff.
She added: "Our first priority is always the wellbeing and health of our students and we are continuing to monitor the situation closely."
The suspected mumps cases come months after hundreds of cases were confirmed among university students in Nottingham and amid a decline nationally in the uptake of childhood vaccinations.
Vanessa MacGregor, a consultant in health protection with PHE, said there has been a rise in suspected mumps cases, with teenagers and young people who have not had two doses of the MMR vaccination "particularly vulnerable".
"Young adults who have not received the MMR or only received one dose should ensure that they take up the offer of MMR vaccination," she said.
"However, it's not just students who are at risk and we would urge everyone who hasn't received two doses of MMR vaccine to do so."