Still a student regardless
He was still was a student in the school system regardless of age. There is the rub. A 19 year old senior year student would fall under the same most likely. Not saying it is right, but if you are in the school system, they are responsible for the student.
Frankly, students who turn 18 during the school year should graduate at the semester break or end of the year, which ever is closest to their turning 18 during their senior year. I don’t care if they do play spring or winter sports. Get them off of campus since they are legally considered adults.