Sure, it's a valid question, but an ambiguous one. It isn't at all clear that the only right way to answer "Are class X taller than class Y?" is to compare the mean height of members of X and Y. There are other metrics — for certain purposes, you might want to compare the maxima, the 95th percentiles, or the medians. Depending on why you're asking the question, any of these (or others) could be the right answer to comparing populations.