Well, that leaves skydown in a bit of a quandary. It still sounds fine to me: I do see "three classes" in this context as constituting a time interval, given that most classes in most institutions are taught in time slots of equal length (45 minutes, say). We need a tie-breaker, and I'll be curious to see if the next person to weigh in is BE or AE. Maybe it's more common as an AE construction?I'd only use the second version "... between every three classes."
While "students should have a break every hour" would work, "three classes" doesn't really count as a time interval in the same way and I think it sounds a bit odd, personally.
Interesting. Just out of curiosity, do you have the same issue with sentences like "They go to France every three years" or "He meets with his thesis supervisor every two weeks"? Or do these sound okay, and there's something specific about classes as a time period that causes the problem?The sentence with "every three classes" affords unlikely interpretations that I have to set aside: If you have three classes in a row, do you need a break in each class? Is the break between each of the three? There is only one way to read it that makes sense, after all, but it takes me a little unconscious sorting to get to it.
Who knows: maybe considering "classes" to be a natural measure of time is peculiar to teachers? "I have a coke every hot dog" sounds hilarious, but I can think of other similar constructions that don't: e.g. "do one squat every four lunges."It is classes as a time period or as any kind of measure. Like DonnyB, I have no problem with "a break every three hours" or with the examples you ask about. "I have a coke every hot dog" would slow me down momentarily too. It can only mean one thing, but I would expect to hear "I have a coke with (or after) every hot dog."