-Have you ever been inside a big government facility?
-You wanna hear about the last time I was in a big government facility?
I think I was in high school. That's about as close to prison as it gets.
That's his closest place of seeing prison so far? As in the most similar place to prison?
<<That's his closest place of seeing prison so far>> This is not a very good (clear) sentence. He's saying that a high school is an example of a big government facility, and that it resembles a prison in many respects. In fact, it's as close to being a prison as is possible without actually being a prison.

- A.
Thanks, Avangi!