During the Halloween feast, when the troll is let in, Professor Dumbledore sends the students to their dormitories. However, the troll is said to be in the dungeons, and that is also where the Slytherin dormitories are, meaning Dumbledore has put them directly in harm’s way by sending them there.The logical move would be for every student to stay in the Great Hall, do a roll call, and send prefects/teachers looking for missing students.
And Hogwarts, home to a deadly Forest full of man-eating spiders, a three-headed dog, numerous pyrotechnics, teaching twelve-year olds how to deal with plants that can, and do, knock them out, staying open when people are being petrified and are in danger of their life, staying open when two students have attempts made on their life, allowing students to risk their lives in a tournament against numerous deadly creatures, surrounding the school with soul-destroying beats, permitting students to duel each other, hiring a werewolf, hiring a delusional, paranoid ex-auror (who turns out to be homicidal maniac), allowing Hagrid to teach, allowing Lockhart to teach, and knowingly letting a student attempt to kill the Headmaster, is renowned for it’s emphasis on student safety. Plus, their Slytherins. And for an added bonus, the Dungeons is a big place and the troll was probably easily circumventable.