RENTON, Wash. -- Nearly one-third of the 1,150 students at Lindbergh High School were out sick Tuesday with flu-like symptoms, leaving some classrooms nearly deserted.

School officials say they began noticing the increase in sick calls on Thursday. Back then, symptoms were more stomach-related. But now most students who were out are complaining of more flu-like symptoms.

By Tuesday, the number of sick students totaled 400.

"It's crazy," said student Matt Smith. "There's like eight people in every class I go to."

The health department is working with the school to find out what is ailing the students but there is no indication as of yet that it could be related to the swine flu outbreak. So far, all cases have been mild and no one has been hospitalized.

School will remain open, and district officials are bringing in extra custodial staff to disinfect the classrooms.

Parents were sent a letter by school officials urging them to keep their children home at the first sign of being sick.