I feel your pain.
At our small Christian high school, our 350 kids are unable to schedule freely as they would in a larger one.
I have never been able to enforce an 'intro to the arts' type course. However, we do have an 8th grade art class which is designed to be a summary and intro to HS. That helps. We of course have many different classes and all of the art teachers make sure that the first unit is basic to the broader arts issues.
We can seem to do this however: we do not schedule beginners into the advanced classes. So that helps.
Somehow this has actually worked to help our program stay healthy, I think, because students immediately get into the photography, ceramics, fibers, or whatever they were interested in. So maybe play that up?