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
Philippa has the weather update for the east
Philippa has the weather update for the west
A ceremony has been held in Portsmouth aboard HMS Victory to mark the 212th anniversary of the Battle of Trafalgar.