If you're looking for maximum exposure, breezeways may be best. You may get 200 kids through a classroom in a day, but you may have 2,000 students walk down a hallway, which is way more competition, but also a lot more traffic. Don't wait until the day before or it'll be too late to attract much attention. On the other hand, if you start too far ahead of time, you'll get posted over. Sometimes it's a chore, but it can be fun.
Some people are taller than others, while others use wheelchairs.
You can't miss them when you start up the steps. Mounting this type of poster is more difficult than posting traditional flyers, but it gets a lot of attention. Unless they understand what you're trying to accomplish, not every member of your organization should be allowed or asked to put up posters. If they break the rules, it may be your group's reputation that could be tarnished. If someone slaps their posters over yours, remove theirs.