I feel like the gyaru stuff is just to cater to fetishes in Japan, when it's actually totally irrelevant for the plot.
The girl is totally normal but apparently, teacher had to be scared of her, just because she's a "gyaru".
And that's very boring and annoying, honestly.
At least in other comics, those gyaru are acting wild and being mischievous, here there's nothing that makes the girl resemble one, except she doesn't have standart hair color, really? Terrible writing skills.