Baggy Trousers was the original title for The Inbetweeners and name of the shows pilot which was produced in 2006 under the direction of James Bobin. All that is known about Baggy Trousers comes from a 1:17 video uploaded to Vimeo in 2013 by Keith Watts who worked on the pilots artwork.
The pilot features five named characters played by different actors from the main series.
Alex Macqueen, who played Kevin Sutherland in the main series appeared in the pilot in an unidentified role.
The pilot appears to have the same script to Bunk Off as Simon is shown spraying 'I love Carli D'Amato' on her drive. William is also shown wearing a suit presumably to buy alcohol like in Bunk Off.
Laura Aikman, who played Carli, appeared briefly in Bunk Off when William talks about staring at girls in the park. It is likely this was re-used footage from the pilot.
The name Baggy Trousers likely comes from the sagging fashion culture.
There is no known character called Jay in the pilot and he was replaced by a black character called Lee. Why Lee's name and ethnicity were changed is not known.