Judge Michael Lawson QC sentenced Forrest to four and a half years for the five offences of sexual activity with a child and one year for the offence of child abduction, to run consecutively.
He told Forrest: "Your behaviour in this period has been motivated by self-interest and has hurt and damaged many people - her family, your family, staff and pupils at the school and respect for teachers everywhere.
"It has damaged you too but that was something you were prepared to risk, you now have to pay that price."
Forrest, wearing a grey suit, white shirt and tie, showed no emotion but nodded to his family as he was taken down to the cells.
More top news
Detectives investigating the murder of Valerie Graves have set up a mobile DNA screening unit in Bosham.
Wednesday morning's weather for the east of the region
Wednesday morning's weather for the west of the region