One of the problems facing schools is the relatively low take-up of some GCSE options in combination with serious funding issues. Teaching GCSE computer science is not a full time position for which they've been able to recruit specialist staff. Therefore what most most schools have had to do is to migrate staff from the old IT curriculum to teaching CS.
I would add that, while working as support staff in a secondary school, I've seen an experienced PE teacher teaching IT very well and an experienced IT professional being a totally useless teacher. It's not all about professional experience in the subject being taught.
Biting the hand that feeds IT
