Kent School Association

A former pupil of Windsor School has told us that in 1990 he, as a 6th former, was taught in the Hostert premises.
His school reports etc are titled "Windsor School".
My understanding is that Kent School officially closed in 1987. I assume that the Hostert site was used by Windsor School for a number of years following Kent's closure.
Does anybody know when the site was completely 'abandoned'?
Thank you.