Foi feita uma pergunta semelhante em Movies & ; TV Stackexchnage e a resposta foi da palavra dos criadores :
When the monster enters our dimension, it’s like a shark breaching the water. Very much like a shark, it drags its prey back into its home, where it feeds. Each time it enters our world, it leaves a small tear, or wound. That’s what Joyce found in her wall in episode 4, and Nancy found in the tree in episode 5. These tears are almost like portholes into the Upside Down. But they don’t last very long. Like wounds, they eventually heal and seal up.
Então o buraco na parede da escola era assim também, uma ferida auto-curativa.