His plea came as neighbours of Forrest claimed he was prone to aggressive outbursts during domestic disputes with his wife. They said police had been called to his family home on several occasions.
Megan was last seen by her family when she left her mother's home in Eastbourne, East Sussex, apparently to attend Bishop Bell School in the town, where Forrest works.
But neither the pupil nor the teacher arrived at school, and CCTV footage from later that evening showed them in Forrest's car in Dover boarding a Calais-bound ferry.
Megan's step-mother Tracy Stammers said the family was devastated by her disappearance. In tears, she said: "This is completely out of character for Megan. We don't know how to deal with this, she should be at home with us.
"She is the most beautiful and loveliest young girl ever and we just want her home safe."
Forrest appeared to have been wooing the youngster for several months through Twitter, including a message declaring: "Me & you. Let's just run away."
On ask.fm, a website where members ask each other questions, Megan was asked four days ago whether she felt excited or curious about her future.
She replied: "Excited, because I know what is to come..." She also said her motto in life was YOLO, an acronym for "You only live once".
Forrest's wife, Emily, 31, recently described in an online post how the couple married in a castle last year and had "never looked back" after moving to their home in Ringmer in East Sussex in 2009.
But a neighbour of the couple, who asked not to be named, yesterday said they had repeatedly fought during their time living there. She said: "They've been here three years and it's been absolute hell because they row constantly.
"Sometimes their rows were so intense I phoned the police. Nothing ever came of it though and the rows continued.
"I'd worry for that schoolgirl's safety if I was her mother. I don't know what happened with his wife, but it didn't sound pleasant. From their rows, I'd say he didn't sound like the kind of person you'd want near your child."
Even Forrest's father was aghast at his son's behaviour and urged him to return Megan to her family.
Jim Forrest, a 59-year-old IT worker, said: "We're all devastated. Everyone in the family is just saying 'what?' They can't believe it.
"In Britain, everyone is innocent until proven guilty, we know that, but first things first, let's get them back. I've tried to contact Jeremy, but his phone is off. I told him 'call me, I know what's going on'.
"Jeremy has always had a real passion for teaching. I once ran a school football team alongside him, and I can tell you categorically, children looked up to him and respected him. He is a popular teacher with students.
"He's a passionate musician and has been in a band since he was 14. A 15-year-old girl has gone missing and we know that this is serious stuff. We're under no illusions what it could mean, but let's get them home and then deal with it."
