Police say Dean D'Amico, who drove a Colonial Intermediate Unit 20 school bus, sexually assaulted the student outside Easton Area High School this past December.
Court documents say the student is on the autism spectrum and has an anxiety disorder, and is almost always non-verbal.
An attorney for Colonial Intermediate Unit 20 told WFMZ the victim was not a student of Easton Area High School, but she was attending one class there.
D'amico allegedly bought the victim a cell phone so they could communicate. The messages were often sexual, police say.
State police arrested D'Amico on Wednesday at his home in Upper Mount Bethel Township.
He was charged with Institutional Sexual Assault, Corruption of Minors, Unlawful Contact with a Minor, and Indecent Assault.
D'amico is being held at Northampton County Prison on $250,000 bail.