<Caveat married to a primary school teacher>
Adding an additional 2 hours teaching per day, and going from an 0800-1730 hrs (in school plus 2-3 hours marking & planning per evening) to an 0700-1900 day for a teacher would MASSIVELY put up teaching costs, because you would need to have, certainly in primary education where most of the important skills are learnt, 2 teachers, and a doubling of teaching assistants, per class to cover the day.
Which would double your manpower costs to deliver primary education.
Also I have children aged 5 & 7, who do afterschool clubs at the moment because Mrs B and I both work, so they are out the house 8-1730 as it is, however the afterschool bit is time for them to socialise and play with "friends" who also go to afterschool club.
It's the last week before half term, and they are knackered, and need the holiday to blow off steam and relax. Formalising the school day to 8-1700 and you'd need shorter half terms to allow the children to manage.
At Mrs B's school they have afterschool clubs 3 days a week that parents pay for, which are sports clubs, massively oversubscribed. Mrs B is also (free) Clubs Co-ordinator for her school, and for every place that she manages she has 3 applicants because a large proportion of parents see it as free additional childcare.
We need to decide if we want schools to be a national childcare service so everyone can contribute to the GDP of the Nation, or whether they want a good quality education service. Private schools spend double per pupil that state schools are paid, the only way we will get a better education system is to spend more per pupil, if state schools are to be compared to private education institutions.