POMFRET, MD., SEPT. 9 -- 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 bleachers} started coming down on top of us," said Jennifer Taylor, a 16-year-old junior who was not injured. "People were piled on top of each other, holding their legs and ankles."

The accident occurred about 8:15 a.m. at the school about 15 miles south of Washington, as principal Herman Murrell was preparing to welcome the new junior class.

Murrell likened the bleachers collapse to an accordian closing in on itself.

"The bleachers gave way all of a sudden," Murrell said.

Four students suffering from back, head and other injuries were flown by U.S. Park Police helicopter to Prince George's Hospital Center in Cheverly. None of the injuries was life-threatening, hospital officials said.

The students admitted to Prince George's Hospital Center were Katherine Irwin, 16, who had a compression fracture of the vertebrae; Rochelle Miller, 15, a compression fracture of the vertebrae; Felicia Brown, 15, a concussion and a sprained ankle; and Christie Stevens, 16, a bruised shoulder.

An additional 74 students, with injuries ranging from cuts and bruises to fractures, were taken to Physicians' Memorial Hospital in nearby La Plata and to Southern Maryland Hospital Center in Clinton. Of those, 12 students were admitted, hospital authorities said.

More than 120 students were on the section of bleachers that collapsed, school officials said. Although some students were walking on the bleachers to take seats, there were no indications that they were overly rowdy.

The cause of the collapse was unclear late today as teams from the Maryland Occupational Health and Safety Administration and the Charles County Sheriff's Department began investigations.

Sgt. Michael Gartland of the Charles County Sheriff's Department said that a "weight shift" apparently caused the bleacher to collapse.

"For some reason, the bleachers must have shifted to one side and with that amount of weight on them, they just kept going," Gartland said.

Gartland said that inspectors will have to lift the bleachers with a small crane and examine the underside before they can determine exactly what caused the weight shift.

Linda Dent-Brown, a spokeswoman for the school, said that the bleachers were last inspected in December 1986 and that repairs were performed in January 1987. The school's policy is to inspect the bleachers every other year, Dent-Brown said.

The school's inspection policy, however, apparently is at odds with the manufacturer's recommendations. The president of Interkal Inc., the Kalamazoo, Mich., company that made the bleachers, said that annual maintenance is strongly recommended. Interkal is the second-largest bleacher manufacturing firm in the country, with gross annual sales of $12 to $14 million, according to company information.

"We send notices to all owners of bleachers, including prior models, admonishing them to annually inspect the under-structure of the bleacher," said Interkal President Francis Hubbell. "If they use the data that we send, any good mechanic in the school district can handle the inspection."

Hubbell said the bleachers -- the standard, wall-attached, manually operated type -- were installed in the school 11 years ago.

Such bleachers generally last from 15 to 20 years with regular maintenance, Hubbell said.

Hubbell said the company does not know why the bleachers collapsed but is sending an engineer to the school Monday to inspect the damaged equipment.

Hubbell said he knew of no other similar collapses since he and a group of investors acquired the Interkal company in 1981.

Dent-Brown said that five other Charles County schools have similar Interkal bleachers.

A representative of the Crofton firm that installed the bleachers at McDonough declined to discuss the job. Ben Rye of Gardiner & Gardiner Inc. said that he had been instructed by a Charles County school representative, Joseph Lavorgna, not to talk about the bleachers.

Several students also told reporters that they had been instructed by school officials not to discuss the accident.

Shortly after the accident, school officials established special telephone lines to handle the deluge of calls from frantic parents.

The gymnasium was sealed after the accident. A school dance planned for this weekend was canceled.

Rescue workers praised school officials and students for their calm reaction to the collapse and aftermath today.

Staff writers Keith Harriston and Jeffrey Yorke contributed to this report.