That's a very visibly divisive reward op. So the kids who had time off have to wear uniform while the majority don't? That's shameful - both that the school is doing it and for the poor kids.
It's not just physical illness either, what about mental health? My 6yo dd is neurodivergent. As a result, she suffers from heightened anxiety and had a period of emotionally based school avoidance after last Christmas. One of her big triggers is looking or feeling different to the others in her class. So she'd have gone back to school (after weeks of struggling with clothing, particularly uniform), then been shamed, publicly, by being in the minority who had to wear uniform on the 'reward' day. Added to this, she has performance anxiety - is constantly striving for perfection and attempts to avoid any situations in which she may experience 'failure'. This would highlight her 'failure' and likely trigger another phase of school avoidance. This absolutely stinks, and if it was my dd's school I'd be kicking up a right fuss.
Certificates handed out as part of a celebration assembly that rewards effort as well as achievement, and includes all for various reasons, then fine (although i'm still not a big fan of rewarding attendance). But really, the reward you cite is particularly cruel and just wrong.
And before anyone starts going on about children like mine needing to 'build their resilience because life isn't fair and they'll need to learn to cope' - we now have an ehcp in place which seeks to help us and school do exactly that. Nowhere in that document does the educational psychologist suggest a strategy of focusing on achievement-based 'reward' and public shaming.
I'm actually really cross about what you describe op.