Following up belatedly on my initial comment on this post (no time this morning to draft an actual answer), how about at just the right level?
At just the right level means not at too high a level, so that many students don't learn anything. It also means at not too low a level, so that many students don't learn as much as they could ... and should.
One should consider at just the right level from the perspective of the students. After all, learning is about the students, not the teacher. That is, what matters is that the students, not the teacher, think the information was presented at just the right level. It wouldn't mean anything from an educational point of view for the teacher to think that all the information was presented at just the right level, only to have many, most, or all of the students strongly disagree.
Imagine surveying the students by asking them to respond to the following statement, with the possible answers being "Strongly Agree", "Somewhat Agree", "Neither Agree Nor Disagree", "Somewhat Disagree", and "Strongly Disagree": "The information in this course was presented at a level conducive to my learning the material." (NOTE: I may not have worded this statement in the best possible way; wording survey questions is tricky. I am using it solely for illustrative purposes.)
The higher the score on this statement, all other things being equal, the closer the teacher is to presenting the information at just the right level.
Of course, there will always be a distribution of responses to such a statement. For example, some students may strongly disagree, either because they considered the level too high or they considered the level too low. It's hard, if not impossible, to achieve perfection when teaching large groups, but there is still an "at just the right level" for the group as a whole.