Good question. If I were well versed in a particular science, such as biology or chemistry, and were a teacher in the public schools, I would first see to it that any textbook used in my class were selected on the basis of its acceptance of open-inquiry. Textbooks that proclaim "billions of years ago yadayadayada" without interjecting qualifiers are misleading, and would either be discarded from consideration or edited, or pointed out as such.
As the given discipline runs its course making use of scientific methods, I might introduce questions that allow the student to explain their observations in both naturalist and creationist terms, or whichever they might prefer. If a student would happen to launch into a sermon exhorting me to repent of my beer drinking ways, I would simply have to remaind that student that God created beer, too, and carry one with the discipline the class is intened to explore.
As far as I can see, evolutionists are on solid ground in maintaining that facts and observations should be the main fare in science classes. At the same time, they should be willing to allow what they call "facts and observations" to be challenged by other ways of explaining them. I mean, when even our best scientists are still uncertain of how to fully comprehend such ubiquities as light, energy, and time, it does little good to snag on creationism as if it will jump out of the bushes and strangle objective realties.
It would serve no purpose to continually launch into the merits or lack thereof of either creationist (why belabor the obvious?) or evolutionist assumptions. I doubt there are many science teachers who hold the evolutionist view and continually bash their students over the head with it. It does no good to the learning process and does not change or advance the facts.
So, you're absolutely right. Science classes should not be made a platform for creationist sermonizing, any more than they should be made platforms for evolutionist atheistic tendencies.
You can state that the moon is made of green cheese also, but that does not make it true.