I don't think it's random, I think there's a repetitive order to the wardrobe she wears on stage. I think the schoolgirl outfit is the last in the cycle before it repeats. If you open the cheat in the Regalia mod, she has a stat that's "Exhi Strip Outfit Count", and I think that just increases by 1 every time you watch her show until it gets to however many outfits she has (5? 6?), and then bounces back to 1.
I could be wrong, but it seemed like a pretty regular pattern every time I've played through.