If there were only three boys out of a hatch of 10 chicks, you were very lucky.
This is a perpetual problem when hatching chicks, and all small-scale poultry keepers struggle with it. The experience of incubating and hatching chicks in classrooms can be very rewarding, but I do wish that teachers would think it through and not embark on such projects unless they're poultry keepers themselves and are prepared to look after all the resulting birds, which sadly, usually means dispatching surplus cockerels because nobody wants them. There would be an outcry if they were randomly breeding puppies or kittens in this way, wouldn't there? And I think the children should know that this was part of the process, and that chicks are not just fluffy toys and that when breeding any animal you have to be prepared to take responsibility for it thereafter.
Please don't feel I'm being in any way critical of you - you're being very kind in taking in these rescued birds, and I can see you are in a difficult situation, not of your own making, I feel. Of course you've become very fond of the boys, but I fear you may have some hard decisions ahead. As Rick says, the relatively small number of people who want a new cockerel would prefer him to be adult, as otherwise he would be bullied by his hens and not able to take charge of them properly. And even if you keep your boys until they're grown up, they will probably start to fight as they mature, and there will be no more likelihood of rehoming them than there is at present.
If anybody you don't know, or anyone not known to us on this forum, does offer to 'give them a lovely home in the country' do check them out very carefully. A lot of cock fighting still goes on, especially in the Midlands, and the perpetrators are always on the lookout for unwanted cockerels, either to fight or worse, to act as bait when training older, fiercer birds.
I do hope you manage to rehome them satisfactorily, and you might even be able to keep one if the neighbours don't object, but keeping more than one would be unwise. if no joy with rehoming, I would advise either getting an experienced poultry keeper to cull them for you, or take them to the vet to be PTS. I would send the bill to the school that hatched them, maybe.