Dennistoun was a student, segregated to a school where he would not age.
Dennistoun came from a society so advanced that it could predict when a child would commit a violent crime decades in his future. They removed these children, of whom Dennistoun was one, from society and placed them in a school where they would not age.
After the boys had been at the school an unknown number of years, a corporeal expression of their future deeds began roaming the school, killing the boys in a violent fashion.
When the Fifth Doctor arrived at the school, Parkins explained to him the nature of the school. Dennistoun then helped his classmates conjure up a classmate out of the evil in their natures, which destroyed the entity that had been killing his classmates.