Not long ago, I was in a dining establishment with a certain colleague. We were not particularly close to campus, but current and former students are everywhere, so it was not surprising when a young man walked over to our table and informed my colleague that he had taken my colleague's large Science 101 class 5 years ago.
The young man went on to enthuse about what a great class it had been and how much he'd learned. Nice!
When I have been in similar situations, I just say thanks, I'm glad you liked it, it's a fun class to teach, and I ask what they are up to these days etc. My colleague said some of those things, but then he went on to muse about how he really knew very little about the topic he'd been teaching and in fact had had to do a huge amount of work to get up to speed on the material in order to be able to teach it.
This was mostly true. The department chair at the time had a penchant for giving this colleague rather onerous teaching assignments, including several new courses and this particular course, which was outside my colleague's research expertise. [For more on the general topic of teaching outside one's expertise, see an old post about whether faculty can/should be able to teach any course in their general field]
My colleague did a huge amount of preparation work when teaching this course. When he thinks about teaching that course, that is what he remembers. So he shared that with his former student.
I have absolutely no problem admitting to a student or an entire class that I don't know something or that I am not the world's #1 authority on a topic. There are various ways to admit to lack of knowledge; e.g., "I don't know but let's see if we can figure it out", "I don't know but here are some possible ideas/explanations", or "I don't know but I know how we can find out [explain]".
I don't think, however, that I would say or imply to a class or student, even after the fact, that I was just one step ahead of them in terms of the subject material. Although that's how my colleague felt at times, that perception ignores the fact that he had a lot of expertise in related fields, and this broad knowledge allowed him to learn what he needed in order to teach the class well. Giving the impression that he just read ahead in the textbook so he could teach the next set of facts or concepts misrepresented his level of expertise.
In fact, the student looked a bit startled when informed that his professor actually felt like he didn't know what he was talking about some/much of the time.
We don't have to pretend to be all-knowing infallible Science Gods, but at the same time, we shouldn't sell ourselves short in terms of what we know. It might be kind of interesting for some students to know what goes into teaching a course -- i.e., most of us don't just stand up there with little or no preparation and empty our brains on the topic o' the day, even in our field of expertise -- but we also should recognize that our years of experience do count for something and help us when asked to teach something a bit outside our research specialty.