I'm just a parent and I don't know what to think about this but thought I would say that as a parent not working in education the school hours & holidays from my perspective aren't always a great fit and take a lot of juggling (and have created problems for me at work - as in having to bring my kids to work when school closed for broken boiler/ inset days/ snow days).
OK I can understand teachers are concerned that their working day will extend beyond 7:30 - 4:30 (which as I understand is fairly close to a 'normal' working day from teachers at our school) but I suspect, in reality, extending the school day for pupils is about convenience for families where both parents work or the only parent works (which probably is now the majority situation, rather than minority).
Like any 'new idea' being floated (and my understanding is Gove's asked the independent School Teachers' Review Body to look into this from what the Guardian reports here: www.guardian.co.uk/politics/2013/apr/18/michael-gove-longer-school-day-holidays - this is about trying to align the school day (which from a parental perspective is 9 - 3/ 3:30) to something more similar to the working day.
Length of day: well many people have infants in nursery 9 - 5 or longer and they seem happy and appear to thrive. A mixture of activities, rest times and meals/ snacks gives them an eventful and nurturing environment whilst Mum/ Dad are off at work.
I suspect that if the extension to the day is overseen by additional teaching staff/ non-teaching staff the impact on teachers would most likely not be in their overall working hours, but would be in a new schedule of holidays & possibly children in school during inset days.
I also suspect that a bit of clever & community orientated thinking may mean that groups like girl guides/ boy scouts, dance clubs, karate clubs, etc... can be invited to host clubs through the schoool. It may also be sensible to have local music teachers consider offering lessons in school. Opportunities like inviting in performances could be encouraged.
I don't know what will come of this idea (this isn't the first time it has made the papers - Blair suggested similar back in the day www.tes.co.uk/article.aspx?storycode=332616) - but right now it's just an idea. If it doesn't actually involve teachers in 'more work' but does mean you have more terms in the school year and more frequent, but shorter breaks - that may be different but doesn't seem unworkable. If ultimately it improves student performance and helps working parents maybe it's worth considering?