From an interview with the Duffer brothers (writers and creators of the show), these portals are explicitly stated to be temporary.
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
It is only the Gate (the original portal) that has not healed (in fact, it grew wider and spread).
So, the portal sealed up after some time, which explains why it went unnoticed by the school's students/staff.