First, I could see it be being done when classes are first being discussed. The discussion could revolve around how to use classes in a semantic way rather than a more layout focused way, where the instructor would introduce Microformats as one option. The value you would be, the students would learn early on about Microformats and have the chance to start using them right away. The draw back is that I could easily become overwhelming trying to learn all that at once.

The second would be once all the basics of HTML are have been taught, having a section where the course would talk about adding more semantic value to the markup. In this instance I think the course could delve deeper into the topic. By this point I, if its a longer running course–like a university course–the students should have a decent grasp on HTML. The disadvantage would be they aren’t learning it from the start, and they could be learning some bad other bad habits on their own.

Third, do a combination of the two. You could end up having the disadvantages of both, while also having the positives of both.

It would all depend on the students, and how much they can take in at once. The best thing to do is try out different methods and see what works.