(CBS) — A sure sign that flu season is here: One suburban school is closed for the day Thursday because there were so many absences.

Robinson Elementary in Lyons District 103 called off all classes and after-school activities because so many were out with the flu.

“We had 80 students who had either called in sick, went home sick, or they weren’t sick, but parents picked them up to go for an appointment,” District Supt. Mary Jo Vladika tells CBS 2’s Derrick Blakley.

That’s nearly one-third of the student body, which numbers 260.

The prescription for Robinson: a thorough school-scrubbing. Classes will resume Friday.

Crews will do the same intensive cleaning at Washington Middle School, in case older siblings of Robinson students bring the bug with them there.

“I thought it was a good idea … so nobody else got sick,” Robinson parent Anngie Kohler said.

The flu scare at Robinson School is a reminder of the importance of being immunized against seasonal flu, officials say.

The state Department of Public Health reports that while most patients recover from the flu in one to two weeks, influenza and pneumonia have killed an average of 3,500 Illinoisans a year over the past decade.

More than 4,000 deaths were recorded in 1993.  Those at greatest risk of serious complications if they get the flu are the very young, the very old and the chronically ill.