Set the alarm on your phone to 11:00 PM, and exit out to the halls. Walk around until you get a cutscene of Paige approaching. Be quick and find a hiding spot - I think there are nine of them in total, maybe a couple I didn't find. (You'll know it's a hiding spot as the cursor will turn into a light blue version of the Nancy Drew silhouette logo!)
Once you've hidden, you'll see Paige pass by. (Usually you just see the light from her flashlight appear and disappear, as well as hear her voice, but you can also see parts of her silhouette from at least one hiding spot!)
Once she and her flashlight light is gone, you can pop back out. Then sneak around some more and do it again!
I don't know the exact number needed, but successfully hiding from her 10 times total was enough for me to unlock the achievement. So do that, and then play through the game to the end, where all the achievements unlock.
(Tip: Save after each successful hide!)