It deopends exactly what is meant by the proposal.
All (or some within an area with lots of schools) school buildings open untuil 6pm, providing a wide range of optional, high quality, registered childcare and extra-curricular activities run by the school or by third parties at a reasonable cost (subsidised for low income families) - I don't have an issue with this. Looking around locally, many primary schools already do this - often with extra-curricular clubs provided by school staff straight after the school day sliding seamlessly into more low key childcare (often combined with homework clubs) on the school premises or nearly later in the afternoon.
Full-on 'teaching and learning', compulsory, teacher-in-front-of-the class school all the way through till 6 - no. For one thing, many children participate in very high quality extra-curricular activities in the community - 1 to 1 music lessons, dance schools, professional club football academies, tennis coaching, swimming lessons / clubs, county oer higher level sport - in the hours between 3.15 and 6, and these could not be replicated adequately in all the individual schools. Shifting all of these beyond 6 pm would deny many children the opportunity to e.g. learn to swim well, learn a minority instrument or play in an orchestra, train non-vocatonally at dance etc etc. For another, the teaching and learning happening would inevitabley be of lower quality. If every teacher teaches 9-6, there is simply no time to prepare and deliver (let alone mark and assess) the type of high-quality and engaging lessons that really make a difference to what children learn. It is quality, not quantity, that helps children to make the greatest progress.
yesm, there are priovate schools that are open much later - my own school day used to run from 8.25 to 6.45. But within that timeeveryonei had 2+ hours of PE per day while my academic teachers were preparing / marking, there was minimally supervised homework and all extra-curricular stuff was on site and within those hours. It is a model that simply cannot be replicated in every school nationwide.