I encountered a bug where the Polyjuice Potion was not unlocked. I watched the "Polyjuice Potion Intro" video, which unlocked the potion, but I've re-entered the Potions classroom after completing the game and now can't find the exit.
I'm going to have to issue a mea culpa here. When I answered your previous question, I found a lot of people who watched the video and then were stuck in the potions room, but there were also a few that made it sound like they were able to continue playing the game. I assumed that the people who were stuck had not properly activated the pictures that open the exit, so I advised you to watch the video to solve your problem.
I have now spent even more time going through a bunch of gaming forums, only to discover that the huge majority of people are actually stuck in the potions room after watching the video, with no way out. The few that were able to continue seemed to be people who had discovered the Polyjuice Potion problem before finishing the game - likely before the Potions classroom changes to the one with the Aging potion in it.
To make a long story short, I'm afraid you are stuck in the Potions classroom and unable to complete any of the Hogwarts part of the game without starting a new game.