I did it many times.
First of all, let me say that just about every meeting I attended was a sacrifice for me. I was a follower, not more. But sometimes, I really didn't feel like going, but I went anyways, just so that the elders would see my face. Usually after an hour, I was bored and fed up. Often I waited for specific time to leave, the timing had to be good...No eyes had to be on me, while I got up and left. I usually brought just a watchtower and a bible and song book, no book bag, making it easier to leave without being noticed....
Usually I waited for the brother reading the watchtower to start reading, then I knew everyone had their eyes on their watchtower magazine, so that's when I left, unnoticed.
A couple of times I acted as if I was just going to the toilet, and left my bible and watchtower on my chair, and just went home, unnoticed.
2 hour meetings are too long, and it's surprising that not many poeple leff half way through the meetings. I guess many would have liked to leave, but just didn't dare.....