Only read half the thread, as it's too long but I think that most people on here saying it's not discrimination and it doesn't matter because other children don't win other prizes (sports, maths) for the most part can't have any experience of having a disabled child or of disability themselves.
When you are disabled, especially when you're in school, your whole life can feel about what you can't do. There are so many ways in which you are excluded.
Often, you can't do sports lessons (I had to just sit and watch them for years. That's 3-4 times a week for 1-2 hours each time sitting there watching children running about giggling, having fun. I wasn't allowed to go to the library instead, for example). You can't necessarily go on school trips, or if you do go, you have to watch from the sidelines (on an adventure day one teacher saw me looking on and said 'you can't join in but you might at least smile and pretend you're having fun. Why you have to look so miserable I don't know,' when I was grieving for my health and desperately wanting to join in). Friends who used to like you suddenly find reasons they can't spend time with you (my best friend aged 12 organised a quad biking birthday party and invited everyone but me. I wanted to go just to watch them and eat cake with them afterwards but she said because I couldn't go on a quad bike she didn't think I'd want to go. As a child, that was devastating). Or your friendships break down because you're never there, for example if you're in hospital a lot. And because of hospital appointments/hospital stays you can be constantly behind on my work too, when no teacher has time to help you catch up and your friends thought it a chore to explain anything.
So you have all that missing out, all that feeling different from your friends, all the wondering why you have to be different when you just want to be normal, then add to that the pain/fatigue/other difficulties which might come from your disability, and then on top of that you have to sit there and applaud other children being congratulated/rewarded for their attendance. Which as other people have said, is down to luck mostly. If you're healthy you are lucky.
This is why it's discrimination. If your average healthy child doesn't win a maths prize, they might win an English one, they might have the confidence to say 'oh well, I'm good at sports.' They might get a reward for effort. The disabled child is already coming from a position where they miss out all the time, every day sometimes. It was a minor thing in my life to watch other children get attendance certificates when I'd have given my eye teeth to be well and at school every day, but it still fucking hurt. I felt like people were saying that I was weak because I was unwell, and that healthy people were stronger and tougher than me. That message took a lot of time and work to unstick.
Some people have no clue how hard it can be for some disabled people. I appreciate some manage just fine, aren't excluded, don't feel different, etc.