As many have pointed out, schools are for education not childcare.
From the staffing standpoint, who will supervise them for these longer hours? Teachers are already burning out, with most working many hours more than full time to keep on top of lesson planning and marking. Contact hours are already much greater than in many countries, and attempting to increase them significantly seems like a recipe for causing more to quit and drying up recruitment further.
From the pupils' point of view, increasing hours in the classroom would pretty much inevitably leave children bored with consequent discipline issues. They're children, and need time out of the classroom to play and socialise.
Even in countries where the school day superficially appears longer than in the UK, the number of hours of lessons doesn't approach a full time job for an adult. For example, French schools are open longer than hours - perhaps 08:30 to 16:30, so similar to a full time job - but primary schools at least have half days or close entirely on Wednesdays, and lunch breaks are much longer - 1.5 or even 2 hours.
The one good point I could see would be if extra hours in school were spent on well thought out extra curricular activities. I don't mean extra music lessons or PE, but genuinely engaging clubs etc. that the children could choose and wanted to engage with. The cost of funding and staffing them would not be trivial, and trying to do it on the cheap wouldn't work out well. You couldn't reasonably use teachers to supervise these, for example, without relieving workloads significantly in other areas.