Hi Pandora:
I believe pretests can be extremly valuable.
I agree that if possible, students should be informed about the level and difficulty of a course, and I think most students are at least willing to try. Sometimes, even under the best of circumstances and selection, students have difficulty and require extra time, tutoring, or assistance with certain ideas or concepts.
It is here that pretesting can be of value. These don't have to be lengthly or time consuming. How valuable it would be if a teacher knew that class time didn't have to be spent on one topic, but… >>>