The 22-year-old, who plays Carli D'Amato in the E4 sitcom, claimed that the upcoming big-screen adventure will likely conclude the series, despite reports of two future TV specials and rumours of a second film.
"If anyone had told us four years ago that we'd be doing a movie version of The Inbetweeners, we'd have laughed in their face," she told The Mail On Sunday. "The show's been so much fun to do. I'm really going to miss it now it's finished - well, we're guessing it's finished.
"Nothing's set in stone but the film kind of ties everything up neatly, and we can't all be stuck in the sixth form when we're getting into our mid-to-late 20s. But it's been four great years of my life... I'm already feeling bereft."
Head also teased that The Inbetweeners Movie will delve deeper into Carli's complicated relationship with Simon (Joe Thomas), and hopefully convince audiences that her character is more than just "a complete bitch".
"The Simon-Carli unrequited crush thing has been great fun to play," she said. "A lot of people think Carli is a complete bitch but I don't think so - I have to like her, or I couldn't play her.
"It's simply that she's exasperated because Simon is such an idiot around her. She never sees the sweet, lovelorn side of him - she just sees him throwing up over her brother, or trying to woo her in urine-soaked shoes."
The Inbetweeners writer Damon Beesley said in June that the movie will serve as the "final piece of the jigsaw" for the show.
The Inbetweeners Movie will hit UK cinemas on August 19.
> 'The Inbetweeners Movie' poster released
> James Buckley: 'Britain can be proud of Inbetweeners Movie'
Watch The Inbetweeners Movie trailer below: