Mr Forrest is a maths teacher at the school that Megan attends, Bishop Bell C of E School in Eastbourne, East Sussex.
The alarm was raised at midday on Friday after Megan failed to turn up for school.
Chief Inspector Jason Tingley from Sussex Police said: "We believe that you left on your own accord with Jeremy but we need to hear that you are OK. Your family and friends are extremely worried, but you are not in any trouble.
"My message to Mr Forrest is that this may not have gone as you expected it to. Right now our priority it to know that Megan is safe and I would ask you to do the right thing and make contact with us.
"We will work with you to get you both safely back into the country."