In the state maintained secondary schools the Remove class was a class for pupils who had already moved through Fifth Form (Year 11) but needed to resit the Ordinary Level ('O'level) GCE examination.
The name originally described the pupils who had been removed (moved on an academic year) from the class described as the Shell.
The latter name originates from Westminster School where junior pupils were taught in an alcove which resembled a shell.
Over time usage evolved differently across individual schools.
Examples at various schools: In Frank Richards' Billy Bunter series the Remove Class is the focus for all the stories, where the term is used as a synonym for Lower Fourth.