NEW YORK - New York City has closed three schools in response to a swine flu outbreak that has left an assistant principal in critical condition and sent hundreds of kids home with flu symptoms, in a flare-up of the virus that sent shock waves through the world last month.

Mayor Michael Bloomberg said that four students and the assistant principal have documented cases of swine flu at a Queens middle school. More than 50 students have gone home sick with flulike symptoms at the school, he said. At another middle school in Queens, 241 students were absent Thursday. Dozens more were sick at an elementary school.

The Health Department said the assistant principal from the Susan B. Anthony middle school is on a ventilator, marking the most severe illness in the city from swine flu to date. The students who have fallen ill in this latest surge of illness appear to be experiencing mild symptoms, similar to routine flu.

The assistant principal, identified by colleagues as Mitch Weiner, may have had pre-existing health problems, the mayor said. In many other swine flu cases that turned critical, patients had pre-existing conditions.

The mayor said that the sick assistant principal may have had pre-existing health problems. In many other swine flu cases that turned critical, patients had pre-existing conditions.

Bloomberg said that three schools - with more than 4,000 students altogether - would be closed for at least a week because "there are an unusually high level of flulike illnesses at those schools."

"There are documented cases of H1N1 flu at one of them," the mayor said, using the formal name for swine flu.