Sixth form was a level of education including Year 12, at which points students were typically around 16 years old. (TV: For Tonight We Might Die, Co-Owner of a Lonely Heart) Sixth formers at Coal Hill Academy were not required to wear school uniforms. (TV: For Tonight We Might Die)
According to Rani Chandra, when Clyde Langer was Gavin's age, he would have found sixth formers big and scary. (TV: The Empty Planet)
Luke Smith began sixth form in 2009, but headmaster Haresh Chandra wanted him to take his A levels already in June. The following year, he was accepted into the University of Oxford. (TV: The Nightmare Man)
Clyde Langer was in sixth form by 2011. Steve Wallace, who left school after his GCSEs and did not go on to sixth form, believed that now he was in sixth form, "he thinks he's better than all his mates." (TV: The Curse of Clyde Langer)
In 2014, Adrian Davies wanted to talk to Clara Oswald about The Tempest, and related changes to the sixth form Shakespeare module at Coal Hill School. (TV: The Caretaker)
In 2016, Tanya Adeola, April MacLean, Ram Singh, Charlie Smith, Matteusz Andrzejewski and Rachel Chapman were all sixth formers at Coal Hill Academy, specifically in Year 12. (TV: For Tonight We Might Die) Tanya was in fact 14 years old, (TV: The Coach with the Dragon Tattoo) and had moved up two years due to her intelligence, (TV: For Tonight We Might Die) or possibly three. (TV: Nightvisiting)