Lisa Miller, now 18, proposed to 60-year-old Jim Kerr at the Scottish International Airshow in Ayr last week.
She was a pupil at Woodfarm High School in East Renfrewshire, Scotland, where Jim worked as the caretaker.
The couple say they have been an item since November last year but earlier this week it emerged Mr Kerr was sacked on the spot when the school found out about the relationship.
Lisa has now opened up over their relationship and how she lost her v!rginity to the granddad after buying him a Happy Meal when he left his wife for her.
She told The Sun :
“I bought him a Happy Meal, then got him a cake and a few balloons too. I couldn’t afford much but tried to make it as special as I could.
“Later on we were watching movies, one thing led to another and things got a bit heated.
“It was my first time but there was no pressure and it was all comfortable.”
Local radio station West FM posted a video showing Lisa proposing to Jim while stood beside the beach.
She can be heard saying:
“Jim, we’ve been going out for just under a year and I love you so much. I wouldn’t swap you for the world and, marry me?”
Jim is then shown smiling before quickly responding “Yes, definitely” before the couple lean in for a kiss.