Maybe it was just a bad day — or a bad week — in the St. Mary’s County schools. But on Friday, four students were criminally charged in three separate incidents at one high school. Two middle school students were arrested in still another incident, which happened Wednesday.

In one of Friday’s incidents, a 15-year-old girl attacked another 15-year old girl from behind at Great Mills High School, the county sheriff’s office said. The attacker was charged with assault and with disruption of school activities, the office said.

At the same school, on the same day, the office said, a school resource officer looked into a report that a student had a knife. The 14-year-old girl was found, the sheriff’s office said, and she handed over the knife, which was described as a switchblade.

The girl was charged with a weapons violation, the sheriff’s office said.

On the same day, the school resource officer at Great Mills High investigated after someone set off a fire alarm. It turned out that a 15-year-old boy caused the false alarm, the sheriff’s office said. He was charged with disruption of school activities, the sheriff’s office said.

Great Mills High enrolls about 1,600 students and serves the southern part of the county. The area is both rural and suburban, and is about 50 miles southeast of the District.

Also on Friday, two sheriff’s deputies went to Esperanza Middle School, in Lexington Park, to look into a report that an assault had occurred two days earlier.

The deputies found that during the school day Wednesday, a 14-year-old boy and 13-year-old girl “were playing around” until the boy threw a football, apparently at the girl.

She swung at him in a playful manner, prompting him to grab her breast, the sheriff’s office said. She then tried to grab his genitals, according to the sheriff’s office.

The two were charged with assault and a fourth-degree sex offense, the sheriff’s office said.