Richmond Adult Community College (RACC) will remain on the Clifden site in Twickenham until September next year, when all courses will transfer to the Parkshot campus.
The council will begin developing the back of the Clifden site to prepare for the influx of arrivals to the new Catholic school, St Richard Reynolds College, which will open in September.
A council spokesman said: “We hope to commence work on the Clifden site at the end of March to begin the first phase on the eastern side to provide sufficient accommodation for the first intake of reception children and year 7 children in September 2013.”
Both the school and RACC will share the divided site for 12 months, until the Parkshot site is completed by September 2014.
New building work includes refurbishment of a classroom to create a science lab, building of the art centre and remodelling the Queen Charlotte Hall to introduce a theatre, dance studio, recording studio and music room.