That means the same as 離れてね and 離れたんだ
Old men or people in a high position occationally use な like that almost only in literary works, TV dramas, and so on. We usually don't use it in the real world.
What the principal ment is that 野球を離れて、今は校長だ。
Certainly, 離れてな is possible as a shortened form of 離れてなさい. But it in the case is different on intonation from a shortened form of 離れてなさい. We have to distinguish them by intonation and contexts.
What Debezo said was correct.