Part of the problem is that the definition of a theory differs between common usage and scientific usage.
In common usage "theory" is akin to "guess." This is not the case in science.
A scientific theory is the current best explanation for a set of facts. It has been studied, picked at, criticized, and generally worked over for some time. It has also successfully made predictions. For any given set of facts there generally is only one theory at a time. If new facts arise, it may be necessary to modify, or (rarely), discard a theory.
When you say that it is OK to teach evolution as a "theory" you are probably meaning to teach it as a "guess." But that is not a correct usage. The theory of evolution is currently unchallenged in scientific debate. The only challenges are coming from religion, and primarily from fundamentalist religions.
Given this, do you really want the theory of evolution to be taught as a guess, rather than the well-tested and well-established theory that it is? While doing this may make some fundamentalists happy, it would be scientifically incorrect.
“In common usage “theory” is akin to “guess.” This is not the case in science.”
I wouldn’t say “guess” just unproven. I may be Christian however I do not believe that ID or creationism belongs in the science classroom, there is less scientific proof of that than there is of the evolution theory. That would be just as ridiculous as making churches teach biology.