De uma entrevista com os irmãos Duffer ( escritores e criadores do programa), esses portais são explicitamente declarados como temporários.
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.
—Matt and Ross Duffer, "Stranger Things episode 6: How the Duffer Brothers created the monster", Entertainment Weekly
É apenas o Portal (o portal original) que não foi curado (na verdade, ele cresceu e se espalhou ).
Assim, o portal é selado depois de algum tempo, o que explica por que ele passou despercebido pelos alunos / funcionários da escola.