Evolution is a theory, and the aspect of evolution relating to small changes between generations in observably valid.
One unprovable hypothesis that can be drawn from evolution theory is that everything evolved from a single organism.
Another unprovable hypothesis that can be drawn from evolution theory coupled with creationist theory is that God created evolution between generations as a way to create variety between generations rather than exact clones.
If you look at the actual mechanisms in DNA and RNA replication where it's pretty much guaranteed to produce some occasional copying errors but rejects large errors, one could very easily argue that the process was designed to produce evolution.
Ultimatelty, it all comes down to fact versus belief. Just as religious education is taught in most UK schools as "some people believe this, some other people believe this other thing, some other people believe none of this", so should things like evolution as an explanation of the origin of all species, the big bang theory or anything else which is an unprovable hypothesis. The children should be allowed to assess for themselves which is the most credible option.
As someone who grew up through school years as an atheist studying a lot of science, I actually found deep flaws in a lot of things presented as "scientific fact" when I dug into them further and many years later became a Christian in part due to contemplating these things. Science is only science when you create theories that can be tested. If you accept untestable theories as fact, you're just turning them into religion, albeit a godless religion, because it is just predicated on belief.
TLDR: Evolution should be taught but in the correct context - as a theory that explains some of the things we can see around us. However, as soon as the government starts mandating that theories should be taught as fact, we have a problem, because by removing the requirement to actually test the theory, it no longer represents science.