JOHNSTON, Iowa — Officials say four Johnston High School students have been arrested and charged with robbery and the sexual assault of a fellow 15-year-old student.

Officials say an 18-year-old man, a 17-year-old boy and two 15-year-old boys have each been charged with one count of robbery, one count of second-degree sexual abuse and one count of felony assault.

The 18-year-old also faces a more serious assault charge, accused of raping the 15-year-old boy with an object.

The Associated Press generally doesn’t name juveniles charged with a crime. The 18-year-old’s case did not appear in online court records Thursday afternoon that would have shown he’s been formally charged.

All four were arrested at the high school Thursday morning.

Police say the incident happened Sept. 8 at the 18-year-old’s home.