Millions of children in Mexico returned to school on Monday to freshly cleaned and disenfected classrooms following more than two weeks of closure caused by the swine flu outbreak.

An estimated 18 million pupils in primary and secondary schools went back to class, although seven Mexican states postponed reopening for an additional week because of suspected new cases.

On Monday 2,059 cases of H1N1 virus were confirmed in Mexico, with 56 deaths,

Worldwide, at least 61 people have been killed by swine flu around the world, and the World Health Organization has confirmed 4,800 cases, including the first in mainland China. China scrambled Monday to find and quarantine more than 200 people on the infected man's flight from the U.S., though the University of Missouri campus where he had been studying planned no special precautions.

Mexican officials spent days disinfecting schools to ease parents' worries. The Health Ministry distributed 16 pages of guidelines designed to prevent the spread of infection in the classroom.

In Mexico City, children lined up outside the Ignacio L. Vallarta public elementary school so teachers could check students for flu symptoms. Some parents worried schools were opening too soon, but many were also relieved after spending two weeks trying to entertain bored children. Officials said any students with symptoms would be sent home.