The point here is that the college's finance is dependant on the number of students it recruits. Even if Marple Hall had a small sixth form of about 150 students it would still make a considerable impact on the college.
The possibility of a relatively small sixth form developing at a future Marple Hall Academy is a much more realistic prospect. And as mabel points out, the school could become an academy quite quickly - IF it chose to apply. However, raising the capital to build a sixth form centre, in the current financial climate, would take much longer, of course.
But I can't agree that the impact on the college of 150 sixth-formers at Marple Hall would be 'considerable'. Camsfc is a very big college, with about 2,600 full-time equivalent students across all three of its sites. If the numbers were to fall to around 2,400 across two sites (Hibbert Lane having been sold), I suspect that the college woud still be financailly secure - indeed, probably more so than today, because the Hibbert Lane buildings are so inefficient.
As for this:
if it's so flipping good then why are the corporation not shouting how wonderful this will be for the people of MARPLE
I think there's already far too much shouting going on!
The college has set out its intentions clearly in this statement
http://www.camsfc.ac.uk/cmsfc-about.asp?AboID=90. The benefits of the plan to the community are explained in the opening sentence: 'The current 1930s buildings at Hibbert Lane are very expensive, inefficient and inappropriate for education in the 21st century. Their annual maintenance costs continue to escalate – money which should be invested in direct teaching for the benefit of the students.'