It's really impressive how the mangaka's expanding on things in the serialized version.
IIRC, we did get to see Airu the first time through, but only briefly and in idol mode - we didn't get even a hint of her background until just now. And the way it played out - slipping into her old delinquent mode just long enough to deal with those guys, then switching to idol mode to win then over as fans - was brilliant.
And the other girl covering Urumin's eyes was a great touch, and funny to boot.
Great chapter.
Mom (and onee-chan) weren't worried about how well she might do on a problem on the chalkboard - they wanted to make sure she was enjoying herself with good people who cared about her. And she is.