Queen Bee Academy! #5 - Breeding Till Sunrise

Your "isolation" theory is interesting, so I went back and reread the chapter, too.

To begin with, a yandere is abnormally possessive. That's her primary trait.
Her possessiveness can manifest itself as jealousy, but only when other girls
are involved. Tsukushi is certainly possessive, but she never displays jealousy
because there are no other girls hanging around Yuuto.

Are they avoiding him because Tsukushi has somehow discouraged them?

I searched for clues, but there's no indication that Tsukushi is interfering with
Yuuto's social life in any way. However, Yuuto describes himself as a "gloomy
jerk" with a "bad attitude" (page 4, panel 1), and everything he says and does
suggests he knows himself pretty well. He's such a surly prick that I propose
a counter theory: other girls avoid him simply because he's not much fun to
hang around with (which confirms Tsukushi's yandere nature, for she has
irrationally attached herself to Yuuto in spite of his unpleasant demeanor).

@623:
@gammenon:
I'm willing to concede that Tsukushi's behavior is "stalker-ish," but not that it's
"absolutely creepy," because that doesn't fit her character, which is essentially
guileless.

As gammenon notes, she's quite upfront about the changes she's made in herself,
and she's not particularly sneaky about it, except in the manner of someone who's
preparing a pleasant surprise for a friend.

As far as we know, Tsukushi hasn't done anything truly creepy, like steal
Yuuto's dirty underwear, or follow him around in order to spy on the way
he socializes with other girls. She never expresses any jealous feelings
at all, which makes her an exceptionally mild-mannered yandere---
in short, absolutely not creepy.

@623:
I wouldn't say being a yandere is Yakumo's "defining trait," but it's definitely
part of her character, and we get an unmistakable glimpse of it in chapter 4,
on page 14, panel 5. You may dismiss that as "shrug-worthy," but I prefer to
pay close attention to the subtle details in manga (which is often rewarding).

And besides, Yakumo repeatedly makes it clear throughout the series that
she wants Aona all to herself---forever and ever and ever. There is always
a slight whiff of yandere around her, however subtle it may be.

I don't think any of that was creepy stalker behavior. That stuff was done to impress him - nothing wrong with that. Only Tsukushi telling him what she changed and throwing herself at him the way she did was odd. But just that - odd.