When I was in junior high, we all had to pass the presidential physical fitness tests. I had passed them all except the standing broad jump. My teacher finally took me outside and spent almost the whole class period with me, practicing this jump. After quite a bit of time, she just looked at me and said "You can't do this, can you?" I could have told her that before we started. It just wasn't something my body was built for.
We need to realize that students are this way in academics as well. Some are just not built for succeeding academically. I do believe every student can be taught to read and do simple math, but some are just never going to do it well. However, if they're doing it to the best of their ability, I think we need to ease up. Yes, they need to keep practicing and keeping up the skills they have and improving what they can, but they shouldn't be expected to do what they can't.
If my entire school career had been based on the standing broad jump, I'd still be in 7th grade, practicing something that I was never going to do well.