It wouldn't be as simple as bringing in a handful of Mums and unemployed supply teachers for the late afternoon. If you're keeping an entire school open for another 3 hours, you'd need an entire school's worth of teaching staff, TAs, First Aiders, Receptionists, Cleaning supervisors, SLT on site, another member of the site team, another IT manager, an additional safeguarding person. You'd need all of these volunteers to have full access to all the personal data of every student. They'd need to access the behavioural system and have people there to escalate to/enforce sanctions the following day. The First Aiders would need access to all the medical data, safety plans for students who have previously self harmed, knowledge of who is Looked After. And you'd need a full handover ever day. You'd need security passes, key fobs, photocopying cards, system log ins, email addresses.
They'd also have to be accountable for any issues with standards in behaviour and academic achievement. After all, if you're leaving your kid in school for another 15 hours a week, you want them to not be taught things incorrectly. You want them to be taught what they need to pass the boards' criteria.
The simple logistics behind it are prohibitive. And the cost.
Oh, plus as this Covid thing isn't about to disappear, you'd also be introducing another 50-100 contacts into the premises each day. I'm sure the 'volunteers' would be slightly more difficult to find in such large numbers, too - they'd need to be able to afford to live without pay, not have childcare costs of their own, be prepared to work in the toughest schools as well as the best - and if you're a volunteer, there's significantly less reason to be in a place where the buildings are falling apart and behaviour is far beyond the odd tut and 'but she's doing it too!' than somebody who takes it because they've got bills to pay.
I can see TeachFirst wanting to get in on it. And some of the other Free School/Academies being interested, as long as they get paid what they will determine to be the Market Rate. But I don't think it is actually feasible.