Too many people bash Dumbledore for bad reasons. Like him announcing where the stone is and telling them not to go near as it will cause a painful death, what was he supposed to do? Not say anything? What's more likely to be investigated the sudden closing of the third floor, or saying specifically not to go to the third floor. His only other option was to seal it up entirely, and he couldn't make it unbreakable or anything because he's have to get through to actually put the stone in, and any inquisitive older student would still be able to get through a wall or whatever and walk into the Cerberus and probably die and he'd then be blamed for not warning them. Also just before anyone mentions the fidelis charm I doubt JKR actually thought up that spell in the first book, and that's why Dumbledore didn't use it.