Like Owl, I went to a huge high school, so I'm including a video that was made of my school a few years before I went. I think it gives a fairly good sense of the scale of the school, but it's also interesting because of the similarities and differences to prison. The security guard checking people in, for example, is more similar to prisons – and in fact, after this video was made, scanners were installed at the entrance to the school so that students had to scan in or out of the building to track attendance. Unlike prisons, though, movement of students in the hallways and common spaces was fairly unrestricted.
Next I wanted to share a fairly well known media representation of prison – the women's prison in the musical Chicago:
And finally, I found two images – one of a school building and one of a prison building. Which one do you think looks more forboding?