Re: high breakfalls?
High breakfalls are a mandatory skill. One goal of ukemi is to be able to protect yourself even if nage is trying to hurt you.
By the time a student is comfortable with normal rolls, it's appropriate to start teaching "easy" breakfalls, such as the "sliding into home" breakfall where you simply breakfall out of a normal roll. Once they can do that, you can start raising the pivot point by having them roll over a partner's hand. Start with the hand at ankle level and raise it as they become comfortable.
Failed attempts are something to avoid in any kind of ukemi. That's a good way to get hurt. If you're getting lots of failed attempts, you might step back to an easier progression (lower support point, more carefully supported falls, etc.) for a while.
Judoka learn breakfalls from the first day they step on the mat. I think the perception that they are an "advanced" skill may do more harm than good.