I'd wondered why no one used a time-turner the moment they knew a troll was loose. Even if Dumbledore had already used up his hours, another professor could've used some form of priority magical communication to call for aurors to travel six hours into the past, swiftly prepare to deal with a Hogwarts-attacking troll, and teleport to the site. Then I realized that Quirrell could prevent all attempts to stop the troll using time travel by exploiting the restriction against information traveling back more than six hours, i.e. by waiting until six hours after he wanted the attack to start, traveling back six hours, and initiating the attack.