Rescued chicks with limp/ balance issues

Hi, I was given two chicks a few days ago (they look 3-4 weeks old to me, but guy who gave them to me says they are older, but not sure he knows) to adopt/ rescue.
They have been hatched in a primary school in a science project, but they had no need for the chicks. They came to me in a not too good state, seemed underfed, weak. Not sure what they were teaching, that animals are disposable and not need proper care? Anyhow, that is a different topic.

After a few days of TLC they seem a lot more lively. But I did notice that especially one of them walks a bit like a drunk. I am trying to observe whether there is a leg injury, or that there are balance issues. Both chicks seem to have feet with rather weird toes, a bit bent maybe? The feet seem a bit less 'solid' and more pale than that of other chicks I've had.

They eat wel, fine feed and love mealworms, vitamin supplement, and have perked up a bit.

I have of course kept them separate from my other hens, in case they have something contagious. Particularly worried about something like Marek's, which has leg issues too. Would a chick that has that seem ill other from leg problem?

Any thought on what they can have? Bad development of leg caused by malnourishment? Injury on leg by handling? Or a virus like Marek? But why they would catch that in a school I'm not sure. No contact with other birds.

How do I find out? Most important for me is the health of the rest of my flock, so need to make sure. Our vet is not too impressive, in Singapore hard to find a vet that knows his chicken.

thanks, Karien