As students filed onto gymnasium bleachers for an orientation seminar today at Maurice McDonough High School, a set of seats collapsed without warning, injuring 78 students, four of them seriously.

The folding bleachers, similar to those found in some other area schools, tumbled like dominoes, trapping several students beneath the wooden risers at the 12-year-old Charles County school. Those most seriously hurt in the collapse had back injuries, broken arms or legs.

Several students seated on the top row clung to remaining sections of the bleachers before falling about 17 feet to the floor, students involved in the accident said.

"We were just sitting there and all of a sudden {the ...