OXFORD CHARTER TOWNSHIP, Mich. — Students at Oxford High School in suburban Detroit rushed for cover and barricaded classroom doors with chairs when they heard the first gunshots on Tuesday afternoon. Within five minutes, the authorities said, a 15-year-old sophomore at the school had shot 11 people, killing three of his fellow students and leaving others with critical injuries. The authorities identified the dead as Hana St. Juliana, 14; Madisyn Baldwin, 17; and Tate Myre, 16, who died in a sheriff’s squad car while on the way to a hospital. The injured students ranged in age from 14 to 17, officials said, including three who were in critical condition and another who was in serious condition. The only adult who was shot, a 47-year-old teacher, had been discharged from a hospital. The rampage at Oxford High School, north of Detroit in Oakland County, was the deadliest shooting on school property this year, according to Education Week, which tracks such shootings and has reported 28 of them in 2021. Students described frantically hiding in classrooms and fleeing from the school after long minutes of terror. Shaken parents rushed to a local grocery store to reunite with their children. Officials across the country issued statements of sadness and frustration, as area residents announced a vigil and prepared for funerals.