(AP) - New York City has closed three schools inresponse to a swine flu outbreak that has left an assistantprincipal in critical condition and sent hundreds of kids home withflu symptoms, in a flare-up of the virus that sent shock wavesthrough the world last month.

Mayor Michael Bloomberg said that four students and theassistant principal have documented cases of swine flu at a Queensmiddle school. More than 50 students have gone home sick withflulike symptoms there, he said. At another middle school inQueens, 241 students were absent Thursday. Dozens more were sick atan elementary school.

The Health Department said the assistant principal is on aventilator, marking the most severe illness in the city from swineflu to date. The students who have fallen ill in this latest surgeof illness appear to be experiencing mild symptoms, similar toroutine flu.

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

Bloomberg said that three locations - all special educationschools with more than 4,000 students - would be closed for atleast a week because "there are an unusually high level of flulikeillnesses at those schools."

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

New York City's first known cases of swine flu appeared in lateApril, when hundreds of teenagers at a Roman Catholic high schoolin Queens began falling ill following the return of severalstudents from vacations in Mexico, where the outbreak began.

At first, the virus appeared to be moving at breakneck speed. Anestimated 1,000 students, their relatives and staff at the St.Francis Preparatory School fell ill in a matter of days.

City health officials became aware of the outbreak on April 24.The school closed and health officials began bracing for moreillnesses throughout the city.

But the outbreak then seemed to subside. Additional sporadiccases continued to be diagnosed, but the symptoms were nearly allmild. The sick children recovered in short order. St. Francisreopened after being closed for a week.

The middle school with the confirmed cases is two miles from St.Francis.