It was revealed in Book 8, Chapter 3 the Royal Sword Academy is a boys-only school:
After Minajael explains that he dressed as a princess in a red dress for a school play, a student says that they had thought that it was a girl student. Minajael responds, “We’re a boys school,” and there is no way that there would be any female students at RSA.
There does not seem to be any specific line that says, “NBC is an all-boys school,” with nothing said aloud and the unnamed students all being male being the only thing we have to base assumptions on.
A conversation arose in this twstsoku thread about how odd it is that the two top schools in the world would be male-only, with someone suggesting that maybe the word being used to describe them is meant to reference men, while maybe all-girls and co-ed schools are called something else. This would make RSA and NRC the top boys schools, possibly outranked by others.
And this is certainly a possibility: Epel calls his grandmother and great-grandmother “witches,” for example, rather than mages, but according to Lilia this is an archaic form of expression, with witches and wizards now being referred to as “mages” across the board.
(EN seems to use “mages” and “magicians” interchangeably, but in the original game the word 魔法士 is consistent.)
Since the schools are likely fairly old, maybe it’s not impossible that some are still using the traditional naming convention, with all-girls schools being for witches and co-ed schools being a thus far unconfirmed third option?
The specifics of NBC seems to be one of many things that Twst is intentionally leaving open-ended so that users are free to enjoy the various possibilities depending on their own preferences. We may never get a confirmation either way, but maybe we will?
Twisted Wonderland Reference Note 