A 16-year-old Ashland High School student suffered serious head injuries after being struck by a vehicle Thursday morning on Main Street, police said. The female student was walking on the street at 6:30 a.m., presumably on her way to school, when she was hit, said Ashland police Lieutenant Richard Briggs. The student’s head made contact with the windshield of the vehicle, said Lieutenant David Iarussi of the Ashland Fire Department. Her injuries were severe and may have been life-threatening, Iarussi said. The girl was taken by medical helicopter to Beth Israel Deaconess Medical Center in Boston, he said.