People keep saying this, but how is this the case? I haven't started school yet, but I assume that an in-class romance followed by breakup isnt the recipe for disaster that many people mention. First and second year, you can avoid your ex (sit away from them in the lecture hall, etc.). Sure you might be in the same PBL group for a few months, but most people can be grownup enough in those situations to be polite. Third and fourth year you may very well never seem them at all. How many students rotate through each clerkship at a time - a dozen at most, right? The odds of getting a highly synchronized clerkship schedule with your ex is very low.
I can see that having an ex in your class can lead to the occasional awkward situation, but unless the ex is an absolute psycho, it will most likely work out fine.