At Shakopee West Middle School in Shakopee, Minnesota, one-quarter of the students are sick with the flu -- 322 students.
Shakopee mom Kathy Young said, "Half of our hockey team was out of school today because of strep throat and flu-like symptoms."
A choir Christmas concert had to be postponed.
Health experts say this year's flu season hit earlier than usual.
Some 1,300 people have died of the flu so far this year, and federal health officials say there have been at least 2.6 million flu illnesses this year and 23,000 hospitalizations.
Flu can in some cases be very dangerous, turning into deadly sepsis, pneumonia and heart problems. New research published in the New England Journal of Medicine shows adults are six times more likely to have a heart attack within a week after getting the flu.
Doctors recommend avoiding people with the flu because they can breathe the virus out in particles that hover in the air.
Frequent hand washing is also important since the flu virus can live on hard surfaces up to a day.