I don't think taking a molecular biology class in high school would make much difference even in college, not to mention after that--in grad school or at a job. Maybe it would make the first unit of the first college MolBio class somewhat easier.

Yeah, I don't have subject matter knowledge here. Maybe I can think of a better example.

I'm not even sure if that would be a good thing since it may develop a laid-back attitude.

This seems somewhat noncontingent. (I might be missing something, but it appears to me that one could equally well argue that students shouldn't learn prerequisite material for subsequent courses, which seems incorrect).

(And even college classes won't really prepare you for real research/job.)

I think that this is the real issue — it's a more significant tension between building human capital and signaling. But it's harder to make a convincing argument for it without deconstructing the commonly held view that high school and college courses "teach you how to think" in general, and this involves a lengthy digression, which might not hold the attention of the reader.

Moreover, it would be totally useless if the student then realized they wanted to go for some other major after all.

It could help the student realize that he or she wants to do something else sooner rather than later, giving him or her more time later on.