Sorry, Zen Scanner! I meant to come back and answer your question when I wasn't on a smartphone and then I forgot... (I hate typing on my iphone.)
Anyway, rigorous, I suppose I would say means challenging and engaging; in higher grade levels, it's definitely short-hand for true college-preparatory. It's one of those words that we toss around in education a lot, and we kind of know what we're talking about, but then ask us to define it and we go blank.
I think that I've decided, based on conversations here and on mothering.com, as well as from conversations with my own director of curriculum (district where I work), that the curriculum is probably what can be expected in my area, and that I am looking at a bad fit for my kid, not necessarily a terrible curriculum overall.
