Evolution is the scientific theory that currently has the most support among the scientific community, so it makes sense that it should be taught as the scientific theory that currently has the most support. Science classes should also teach that no scientific theory, no matter how well supported or how highly thought of is ever actually proven, and that this applies to evolution (and yes, even to theories of gravity before we get one of those answers).
Intelligent design does not have so much support, so it probably shouldn’t be taught in schools, for the simple reason that non-biologists (meaning not me either) don’t really have the tools to evaluate the details of the arguments involved. In presenting this idea without going through the standard scientific channels, you essentially get a lot of people making uninformed guesses and then arguing about them.
As an example, I could give a standard class of high schoolers who haven’t had a physics class a brief over view of the inherent assumptions of Newtonian Physics, then a brief overview of Quantum Physics or Relativity (using different names so that they wouldn’t just recognize the words and associate them with what they’ve heard is true), and then ask which one they thought was true. Chances are, unless they’ve had outside exposure, right about the time I start telling them that electrons can spin, sort of, but only with one “speed” in each of two directions about any given axis, or that position really isn’t a fundamental quantity as such, but rather there’s a certain probability that you will see the effects of any given object in any particular region of space, I’d lose them all.
Granted, I could site studies and tests, but unless they’ve had exposure to the science, the tests won’t really mean much to them, and they’d have to trust me when I said a certain number showing up on an oscilloscope meant that nearly everything we’d intuitively guess about the universe is wrong.
My point is, at least at the beginning of scientific learning, we have to accept (to a reasonable extent) what we learn on the reasonable authority of our teachers, because we simply don’t have time to reinvent the wheel for every single scientific theory we come across in high school. This places a burden on the teachers to only present as reasonably well supported theories those theories which are in fact reasonably supported by the scientific community as a whole, since chances are we will assume that whatever we learn in school has at least as much weight to it as such a theory would have.