Actor Allusion: Mr. Gilbert, the imposing sadist who openly hates the students and even admits that the sole reason most people become teachers is because they stopped doing background checks, is played by comedian Greg Davies who was a teacher for thirteen years.
Dawson Casting: All four lead characters' actors were, at the youngest, 21 at the time of filming the first series. This means that, if they had left college/sixth form and gone straight to University, they would have graduated from that three years ago. This is one of the few positive examples of the trope, because all four of them sure don't look like they're in their mid-20's.
Well, there is the scene where Will is in bed with Charlotte. No average-sized 16 year old has that much chest hair...
It's rare, but not impossible. Boys who begin puberty at a very early age can sometimes have that amount of body hair and even grow a full beard during their mid-teens.
They were obviously aware of this, as it's gone in the later seasons and movies.
Inverted, however, by most extras being Year 7 students- and not due to begin sixth form for another five years.
The best example: Susie, one of the "freaks" who joins the Sixth Form with Will, is said to be taking her A-levels four years early, making her either twelve or thirteen. The actress playing her was seventeen and actually would have been taking her A-levels.
The age disparity increases with the second film as the gap betweeen the release of the two films is three years, but In-Universe only a few months has passed. Making Simon Bird and Joe Thomas roughly a full decade older than their characters.
Enforced Method Acting: None of the actors who played the audience during the fashion show episode (including Simon's parents and Mr. Gilbert) were prepared for Simon's legendary Wardrobe Malfunction. It's right, none of them knew that when Joe Thomas entered the stage, his left testicle would stick out from his clothing. So any laughing or shock from the audience's side is probably genuine.
Life Imitates Art: Simon (Joe Thomas) and Tara (Hannah Tointon)'s on screen relationship may have ended disastrously, but they are now together off screen.
Name's the Same: A differently spelled first name it may be, but Carli shares quite a lot of similarity with Carly from iCarly, especially considering they both have a Dogged Nice Guy who has a Just Friends crush on them (Simon for Carli, and Freddie for Carly). In the fashion show episode, Carli uses the line "Please for me" on Simon to get him to do something for her, which is identical to a line that Carly constantly used on Freddie to get him to do things she wants. Although the similarity of the two ends the further towards the end of the series and The MovieThe Inbetweeners gets.
Recycled Script: The motorbike scene at the start of Series 3 Episode 3 was originally written for the film but cut and repurposed for TV series.
In the second film, dialogue in some scenes that were cut is reused in other scenes:
Neil's Info Dump about his irritable bowel syndrome originally took place in a scene on the group's flight to Australia.
In a voiceover during the Splash Planet sequence, Will mentions that Jay insisted that the water in the park was "80% vaginal fluid". Jay originally said this about the Lazy River ride in the same scene.
Unintentional Period Piece: Definitely can be seen as one to the late noughties: the soundtrack is taken from that period, mobile phones used by the characters are of the pre-smart phone variety and there are no tablets, characters use PS3s and Nintendo Wiis, social networks in use are Facebook, Myspace and Bebo (The latter two of which have fallen out of favour since), while Twitter, Snapchat and Instagram are not (though Neil uses Grindr in the second movie).
Matt Smith auditioned for Will, but he was deemed too handsome and dashing for the part.
Working Title: The pilot episode was entitled Baggy Trousers. Joe Thomas named the cast's 'band' on the radio after the final show's working title of 1, 2, 3, 4. This was most likely a reference to the song by Feist of the same name, which was something of a Leitmotif during the first series and appears sporadically throughout the rest of the show.
The second movie got a few although they were much more clear cut; The Inbetweeners Down Under, The Inbetweeners Movie 2 and The Inbetweeners 2: The Long Goodbye. The latter was what appeared on all the scripts and (strangely) is the title that appears on Blu-Ray copies when read by the player.