The firm is understood to have initially been up against Bouygues for the job after Hounslow elected to run its own bid competition rather than use the Education Funding Agency framework.
Farrans secured the Gresham Road school, designed by Pick Everard, with a £32.6m bid. One of the key challenges in the design was mitigating external noise with the site so close to Heathrow.
The new Oaklands secondary school and college buildings will have a floor area of 90,000 sq ft over two storeys and are targeted to achieve a BREEAM Excellent rating.
The 224 pupil school for pupils aged 11-25 with severe and multiple learning difficulties is scheduled to open in September 2018 but will not reach full capacity until 2023.