puzzle

[brave sinew]

You have to complete a story mission first. Once you have access to the Headmaster's office, the key to this door is in there

Accio

Well, as long as you got it working, that's the important part

[astral rapids]

Don’t you need Alohamora level 3, because I’ve seen the key shape in an outline from Revelio but I swear the door was locked with level 3 Alohamora

[brave sinew]

No. It's a key in the Headmaster's office

[astral rapids]

I was in his office and I didn’t find a key

I went out onto the balcony and saw a door upstairs and there is a key in there

[brave sinew]

That would be it, yeah

You need access to the office to get that far, though