A good question indeed is how old the Academy, and how much time Wranglum is there. The room where is it seems an abandoned magazine full of forgotten stuff. I'm not surprise if the Academy staff neither remember what they put there.
But also possible that room was really the study of one of the professor, that when disappeared the Academy staff simply "sealed" or left empty! The state of abandonment is just becouse nobody set foot in the study for long time.
And watching the concept arts a second time, the room of wranglum is more a study left empty: the blankets on furnitures are typical to cover them when is knows that these will be not used for long time. Setting it in the basement sounds very close also to put it in a "dungeon", a classic fantasy clichè where a lot of "dark and evil" warlock love lives. ;)
I think then that nobody know what is the destiny of the magician: one day they just find a mirror, and they put it in a chest thinking was one of beloning of Wranglum... without knowing that "it was" Wranglum itself!
Indeed seems also that Battiwig find the mirror in the chest, being surprised to see him "alive".
I think that, much simply, one day they sended Battiwig to the abandoned room to search something (maybe a solution against the "monster"?), and accidentally he open the chest and find the old mirror, awakening it in some way if it was "unactive". The reason why he want help him still a mistery for now.
That also set Battiwig not just the Wranglum "henchbat", but part of the regular Academy staff. Is possible that him interact in some ways to other main characters, but stay in background and keep the secret of his link with Wranglum? That would put Battiwig in a interesting situation where he is a spy, he can move freely in the Academy doing secretely the Wranglum agenda, but also interact with the "good guys" that could, indirectly, make him rethink at some point in the story of his relation with the mirror. What really matter is "why the bat help the mirror?" If they get a very good reason, I think the little bat will turn in one of the most interesting character of the show.
Another possibility, is a thing that Zejgar point out in one of the Filly commercial, where Princess Sparkle keep in "hoof" a mirror that resemble very much Wranglum. What is the Dark Mirror was really a magical object owned by the Princess? If they are like magic wands, is possible that Wranglum tried to steal it to study its magic, but something going wrong and the mirror "suck him".
They have some really interesting stuff in their hand, I must say.