I'm a bit late coming to this thread but I too worked as a developer in industry for many years (C++, VB, SQL, a bit of Java etc) before working in schools. And many of the thoughts and frustrations here speak my mind.
I have several times sat down in beermat design meetings with old mates. The conclusion is that you could nail this product and the whole lucrative market with a small team of maybe 5 people.
The problem is getting it into schools. Look at SIMS' (is it 80% marketshare?) origins as a model.
1) A home school to use as sandbox - OK that's probably achievable
2) A local authoritty that will take it on - No chance: they're mostly 3rd rate battery hens
3) selling power/ schmooze ability / graft - No chance: makes me physically shake with rage
So number 2 is a systemic block - do you know a LA with those balls?
Number 3 is an personal block but also a common shared profile for software engineers. You can't afford parasites on the team so you need that rare engineer who is also a corrupt smarm merchant. And no ethical values.
The solution I think is contained in the reason why the beermat design team meetings always collapsed well before last orders: we always fell out over
SQL Server v MySQL
I always said you want to integrate web parts and Outlook groups so the MS overhead is worth it, but my hardcore mates insisted on full opensourrce.
When I look back now I suspect they were right: The solution needs to be totally free and pupils should be able to help out on the development.
You make the money from installation, support, training and lots of bespoke jobs.
As I've said I'm an engineer and no salesman but I suspect you sell it to the teachers, not the schools or LAs: the dinosaurs are almost extinct - all that's left in schools in 5 years is those 20 something and 30 something eager beavers (soon to be burnouts!) who love computers.
Make it part of some awesome, must-share portal eg: a free national supply teacher agency website.
My 2c worth of big picture...someone else can handle the details, meanwhile I'll enjoy my evenings, weekends and holidays.