The School history shows that it was a Cadet Corps of the 4th Northants Regt. They were wearing a school badge in 1913 and not the Northants badge.
http://www.raundswarmemorials.org/4946/16724.html
This letter from 1915 confirms that the School had a cadet corps rather than an OTC. It had a school detachment in the Army cadet force in WW2 suggesting a cadet corps background.
I can find no on-line record of it becoming an OTC.