I feel like I should comment since there's so much discussion in here as to what translation we should have gone with for "maniacal girl".
There are three main reasons as to why we ended up going for maniacal:
1. Rhymes with magical, so "magical girl" sounds similar to "maniacal girl"
2. The definition of maniacal, "exhibiting extremely wild or violent behavior"
The last reason is a spoiler,
consider yourself warned.
Some of the other words we considered (see credits page) took the "demon lord" thing into account more, but had less visual and audial similarities to "magical".
As for what most of the comments are suggesting, going for Maou Shoujo: we did consider it, but decided against it. The main reason that
I didn't want to go for Maou Shoujo (I can't comment for the rest of the team, because I'm not sure everybody's reasons) is that the club is the "magical girl club", which has been established over all of the previous chapters. Going for Maou Shoujo when we've been translating Mahou Shoujo this whole time would be inconsistent.
The teacher panel would also make way less sense if he was confusing "magical girl club" with "maou shoujo club". It's highly unlikely we would go back and change all of our previous references of "magical girl" to "mahou shoujo" and "magical girl club" to "mahou shoujo club" just to make this one concept sound a bit better.
Unfortunately, this is one of those compromises in translation that we have to make.