UMASS Amherst, an "experimental" elementary school for phycological observation. It's now no longer in operation, but for decades it was an elementary school with an upper "secret" corridor with two-way mirrors overlooking each classroom. The understanding between UMASS and Amherst was "We let you use the elementary school, we let our psych students take a look" type of thing.

Each classroom had a row of two-way mirrors about 30 feet overhead [weird architecture, nature of the beast] so even physically it was out-sight and out-mind because "nobody looks up".