CNN reported that over 1200 people may have been trapped in that school in Russia and over 200 are now dead.
You can read the report by following this link.

I found out why so many people were in that school. Apparently in Russia when it is the first day of school parents,
friends, relatives, etc come out to a sort of first day party. Must help the kids in feeling school is fun when younger.
(from BBC News article - Russia siege goes into third day)

The reason for being naked, (or nearly so) I heard was that it was so hot in the school.