You can get in once you are level 60. I am not totally sure, but you might also need to be done with Wysteria to get in. When you are level 60 and are done with Wysteria (If being done with Wysteria is a requirement) you will see one of those bubbles with a persons face in it (I forgot who) but that person will request you to come to them. There may be a few quests after talking to them, but the person who requests you to come to them will be in Wysteria.
Try going to Wysteria and hit the "Find quest" icon in your quest log... or go see the Headmistress Belladonna Crisp. She's the one who gives the quest and if she is ready to give it to you she'll have a yellow exclamation point over her head.
You can get in once you are level 60. I am not totally sure, but you might also need to be done with Wysteria to get in. When you are level 60 and are done with Wysteria (If being done with Wysteria is a requirement) you will see one of those bubbles with a persons face in it (I forgot who) but that person will request you to come to them. There may be a few quests after talking to them, but the person who requests you to come to them will be in Wysteria.
You should get there at level 70. Ignore the Request bubble at level 60