I believe this scene is a parody of Negima!?, the Shaft reboot of the Negima anime, which I like to refer to as Pani Poni Negi due to its strong stylistic resemblance to Shaft's Pani Poni Dash. (Plus, when you're speaking, it's really hard to distinguish Negima from Negima!?.)
This is a classroom shot from Negima!?
The characters in the shot from Zetsubou-sensei also resemble the characters from Negima!?. In the shot above from Negima!?, the characters aren't all sitting at their usual positions, but of the ones who are, several appear to be in the same position in the Zetsubou-sensei shot. Starting from the top left in the Zetsubou-sensei shot and and going left to right, top to bottom, we seem to have:
- Yuuna Akashi; we can see her trademark side ponytail
- Yue Ayase; her high forehead, small ahoge, and the two braids stand out.
- Haruna Saotome; she has long hair with two small antennae
- ? (Could be Zazie Rainday in glasses)
- Satomi Hakase; she has two thin braids.
- Ku Fei; she has two spiky twin tails.
- Probably Konoka Konoe; she has long dark hair like this character, and as in the Negima!? pic, she's seated next to:
- Asuna Kagurazaka, who has long, thin twin tails tied with bells. The character in the Zetsubou-sensei shot seems to have two little buds on her head, though it's too far away to see if they're bells.
- Misa Kakizaki; the bangs are similar and she's in the same position as she is in the Negima!? pic.
- ? Based on the Negima pic, probably Mana Tatsumiya.
- The Negima!? pic shows Chizuru Naba in this position, but the Zetsubou-sensei character doesn't really resemble her.
- ? I can't identify the character in the Negima!? pic either.
- Misora Kasuga, who has a short, spiky buzz cut.
- Chao Lingshen, who has buns and braids.
- Fuka Narutaki, although the character in the Zetsubou-sensei pic seems to have glasses. But she shares Fuka's pointy twin tails.
- Akira Okouchi, who has a long black ponytail.
- Madoka Kugimiya, who has short hair and stylish angle-cut bangs.
- Setsuna Sakurazaki, who has a spiky ponytail and straight bangs. It's hard to tell who's sitting there in the Negima!? pic since they're turned to the side, but it's pretty clear in the Zetsubou-sensei pic.
- Nodoka Miyazaki, who has short hair and long bangs and sits in the same position in the Negima!? pic.
- Ako Izumi. In the Negima!? anime she had bluish-grey hair like we see in the Zetsubou-sensei pic, although in the manga and other anime she had purple hair. She's the one on the right in this picture from my rather large collection of screenshots that I obsessively gathered for some reason when I was watching Negima!?:
This is the character you were asking about in particular. According to the sound effects (ira ira) she's angry, and I can't really say why. My only guess is that it's a reference to the Negima manga; in the manga, Ako confesses her love to Negi when he's magically aged up to 16 and later suffers heartbreak when she finds out who he really is, so maybe she's offended that Nozomu is implying that she shouldn't have said anything. That's kind of a stretch, though.
- Kaede Nagase, who has a distinctive pyramid cut.
- The short-haired Natsumi Murakami.
- ? The Negima!? pic shows Makie Sasaki but the character in the Zetsubou-sensei shot doesn't look anything like her. She does somewhat resemble bit character Kokone from the Negima manga, though.
- A much taller version of Fumika Narutaki; she retains the trademark buns.
- Sakurako Shiina, who has spiky twin tails and appears at this position in the Negima!? shot.
- Ayaka Yukihiro, whose long blonde locks are unmistakable, and who appears in the same position in the Negima!? pic.
- Kazumi Asakura, who has a spiky ponytail similar to Setsuna.
- Sayo Aisaka, who has long hair.
The only Negima characters who aren't accounted for are Evangeline, Chachamaru, and Chizuru, and none of the mystery characters really resemble them, but otherwise it's a pretty striking match. Plus, we know that Shaft has done this before.