To be fair, loads of constructions sites have such cabins, with a generator, and pretty easy to hook up, but you won't get many kids in them, you need space for them, they wouldn't be very secure for any equipment, and the construction industry will want them...
Not to mention the prices you'd be charged by opportunists.
At a push some schools might be able to use vacated buildings / halls - but less than ideal for children who really need familiarity.
2.5 days a week in two shifts might help (half the numbers), but it would probably be left to the schools to arrange all that, which again, would cause all sorts of upset about billy not being in the same half as freddy, and little Rosie who NEEDS to do Tuesdays and Thursdays etc.
Even half the numbers is still a LOT of kids. 'Pissing in the wind' springs to mind, but maybe only get one shoe soaked.