There does not seem to be any specific line that says, “RSA/NBC is an all-boys school” like we repeatedly get for NRC, but it seems largely believed that RSA is also an all-boys school based on the students we have seen thus far, and from a comment made by Leona in Book 3:
He uses the words 優男 and おぼっちゃま, which describe men, and this was even carried over onto EN with the word “lordlings,” with no mention of anyone but boys being at RSA anywhere in the game.
But, like many things in Twst, it has not technically been confirmed either way (as of this writing).
NBC is even vaguer, with nothing said aloud and the unnamed students all being male being the only thing we have to base assumptions on.
If someone prefers to go in the direction of RSA being co-ed for fanfic purposes, you could reference the fact that our strongest argument against it comes from Leona:
With Ruggie’s line of Leona always being “extra nice to girls,” an argument could be made that he was only insulting the men at the school and chose not to comment on its female students.
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 schools like NBC and RSA seem 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 