I disagree. I'm a teacher myself (high school level, 15-19 years old) and already said to my students "I've been awful during this class, sorry, I'll try to do better" (well, it was worded as "I've been fucking bad"). Your analogy is understandable from a business standpoint, but I don't run a business, I don't need to preserve some image or branding. My job doesn't depend on that. And students like honesty.
Well, I guess it's easier to be 100% honest when your job isn't on the line