Authorities in Calvert County are investigating two separate school bus accidents that occurred yesterday, including an early morning collision that left a bus driver with serious injuries and four scared preschoolers with minor bumps and bruises, police and school officials said.

The driver of a bus taking children to a Head Start program near Port Republic pulled into the path of a dump truck on St. Leonard Road about 7:19 a.m., police said. The impact caused the bus to roll over, ejecting the driver. The bus came to a rest in the intersection of St. Leonard and Parkers Creek roads.

Investigating officers said the accident was probably the fault of the bus driver, Mildred Berry, 45, of Owings.

"The driver apparently didn't see the dump truck coming," said Sgt. William Soper, a spokesman for the Calvert County sheriff's office. "The school bus flipped over and landed in an upright position."

Berry was taken by helicopter to Prince George's Hospital Center, where she remained in critical condition last night. A bus attendant and the four children, who range in age from 3 to 5, were treated for minor injuries at Calvert Memorial Hospital in Prince Frederick and then released. The dump truck driver, Mark Arledge, 33, of Huntingtown, was not injured.

"The children escaped this physically fine, but we would not want to [understate] the trauma of this experience," said Dana Jones, the executive director of Southern Maryland Tri-County Community Action Committee Inc. in Hughesville, a private nonprofit community organization that has operated Head Start programs in Southern Maryland for 35 years. He declined to release the names of the four children or their parents, whom he met with at the hospital shortly after the accident.

Class was dismissed for the day at the Island Creek Head Start Center in Port Republic, where the children were heading at the time of the crash.

At the center yesterday afternoon, teacher Florence Gross, of Prince Frederick, was using the downtime to clean out a classroom closet. Overhead, paper plates decorated with hand prints, pumpkins and student names dangled from the ceiling.

"Of course, we're upset," Gross said. "We knew the bus driver. I know all the children. . . . These children are precious to me. I feel these children are my own."

Jones said Berry had driven the school bus for his agency for two years and had an excellent driving record. According to Maryland driving records, she received a speeding ticket last year for going 64 in a 55 mph speed zone less than 10 miles from the day-care center.

In yesterday's second Calvert County bus accident, a bus carrying 43 Northern Middle and High School students collided about 3 p.m. with a car whose driver allegedly cut in front of the bus near Route 4 and Chaneyville Road in Owings, school officials said.

None of the students on the bus was injured, but 34 students were taken to Calvert Memorial Hospital as a precautionary measure. The nine other students were picked up by their parents.

Staff writer Nancy Trejos contributed to this report.