Teaching everyone with high expectations is a great idea. In practice, though, it means that some people flunk. (I'm not talking about the fact that some students require different instruction; for instance, some students are special education students. I'm talking about standard students.)

Many school systems find that flunking folks requires a disproportionate amount of effort. There are ways to work through that, but in the end there has to be a public will to do it. At least in the US, there is a very large thread of public discourse that thinks the goal in the education system is to minimize the cost of educating other people's kids, not to maximize what they learn.