Hello, I made a video for all the achievements. I speak German but you don't need to hear my voice to follow the video. You can mute me or just activate the translator.
When you are in the school building, follow the hallway to the left until you are in front of the sports hall.
Turn to the left; there is a ladder on the wall you can't interact with the ladder, but you can still jump on top with spacebar, the achievement will pop after you are on top.
Climp 03:37