School should first and foremost concern itself with effective education. If children can only study effectively for 2-3 hours a day, that is how long schoolday should last. Parents that need childcare should seek it elsewhere - that should be outside of the scope of schools' responsibilities.
Currently school is trying to do education, socialization and childcare in one big lump and doing all poorly.
I fear though, that if school was not responsible for childcare, there would be no support at all for childcare for working parents - and we've seen the disastrous childcare crisis already in effect for ages 0-5. So this problem needs attention too.
I wrote more about the idiocy of lumping childcare, education and socialization together here : https://manisharoses.medium.com/not-school-or-homeschooling-...
I have a data point that is perhaps relevant. I went to school in Eastern Europe in the 90s, and my education from secondary school onwards was naturally modular.
Normal day school was from 8:30-12:40 or 8:30-13:30 most days, with homework afterwards. But it was effectively optional - quality of education was tailored to the lowest common denominator, and it was universally understood that you are not getting into any decent university by just attending school. Universities set their own entrance exams, school marks did not matter to them, and they did not care how you got to reach the high entrance bar that they set. So teachers, principals, parents all understood and acknowledged that preparing for entrance exams comes first, that they are probably not qualified enough to prepare you, and so school comes second.
And there were many options for actually learning. Many parents paid for private tutors. But there were also (free) computer clubs, where somebody would volunteer to work with children on programming or 3d modelling projects - some structured, and some unstructured, where you would just turn up, try to create, ask questions and be helped out, progressing at your own pace through your own interest. There were very affordable maths and physics classes by post - where they would mail you textbooks and a timetable, and you would mail back completed homework, receiving feedback. There were evening schools, some of them free, where you would attend lectures and classes in person, mostly geared towards maths and physics, which would give you first class education - if you cared to learn.
If you skipped school or did not do your homework, or were even working on something of your own during classes, it was tolerated by the 'compulsory' schools. So effectively, you were free to pursue your own modular education in the spirit that you describe. And those that wanted to learn, people that I know, did very well out of it.
With regards to the 9-3 schedule, this is one of the great things about homeschooling/unschooling/modular learning. Just like you can design your child's education, you can design their social experiences and their childcare experiences to optimize them for your child and family. School needs to evolve with an evolving work force and 9-3 doesn't make sense.
If you don't live in an urban area, it's totally possible to set up an outdoor work space while your kids play outside.
Kids may be an afterthought for society, but they are definitely NOT for most parents. And we need to be more assertive about making sure that needs as a family are met. For example, it's unacceptable for a huge corporation to design a multi million dollar space with no daycare - and certainly not conducive to hiring talented women to work there who could bring a ton to the company.
Also, parents need to be more aggressive about asking their jobs to let them carve out time for the kids. With remote work it's hard, because when your kid gets sick, it's not like you are out of the office at home with them, you're supposed to be on and tuned in remotely while they are at home with you working.