Magic School

Magic School is an institution of witchcraft started by the Elders following the destruction of Atlantis. Following Gideon's death, Paige Matthews became headmistress before giving the job to the newly mortal Leo Wyatt, who taught there until his retirement.

On the Show
The sisters first found out about magic school in season six.

Earth
There are many portals to get to magic school all over earth. They can only be opened by someone magical and like the portal Paige showed Brent the portals to magic school are nondescript and often look warn down.

According to Leo Rising, a Charmed novel written by Paul Ruditis, the sisters created a portal to Magic School on first floor of the Halliwell Manor after Leo became mortal, given that he could no longer orb and he could not always rely on Paige to orb him. The portal was mentioned in No Rest for the Wicca.

In the Comics
"The Elders established Magic School as a training ground for young magic users. Set apart in a separate realm, hidden portals all over the Earth allow access to the school for those who have permission to enter. The school was set up as a safe environment for learning and protective wards were created to keep the students safe from death during their studies. Those wards were breached when The Charmed Ones went into hiding, allowing Magic School to fall under demon domain until it could be reclaimed."

While Piper, Phoebe, and Paige go along their days, Leo is left watching the kids. Left with six kids, four of which have either no walking ability or questionable walking ability, Leo is unsure how to get them to Magic School.

When the nonmagical innocents turn on them, Phoebe, Paige, and Coop escape to magic school with an unconsious Henry. Leo is already there with the magical innocents. Once there, Henry is no longer infected by the magic that turned the innocents, but he remembers what he tried to do to his wife. When Piper gets home, Paige and Phoebe help her escape the Source's attack and Paige orbs them all back to Magic School where they try to decide what to do next.

When Paige's house caught on fire, Paige took Tam, Kat, and Little Henry to Magic School. When they chased down Montgomery Knox, he pointed out that demons had taken over Magic School, something that should never happen at a school. While they were talking to him, they got a call from Leo telling them to come to Magic School. First Tam and then Kat showed off their powers.