In S02E02 we see a breach created in the Hawkins high school by the Demigorgon, a gate between the real and Upside Down world. Eleven enlarges this gate to enter the real world. What happens to this gate? Clearly regular school continues later, so the gate must not be existing, but then did the gate close? And would that mean that these breaches can be closed?
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.
So the hole in school wall was like that also, a self healing wound.