You're surely right about the difficulty getting the kiddies to understand the role of assumptions -- but, then, economists use assumptions like mathematicians, almost as axioms. Our assumptions represent intentional oversimplification. But, the kiddies who remain obdurate in the face of economic theory usually never much cared for mathematics, either.
The best comment on the study of economic theory I've ever seen came out of the introduction to the second edition of Alchian and Allen's University Economics, published in the early 70's:
[more or less] The economics most professional economists use in their work is the economics they learned as sophomores. All the work they've done subsequently through graduate school is to convince themselves that what they learned as sophomores is correct..