Makoto and Ibuki were both about 20 in SF3. They're a bit younger in SF4 because it's a prequel, but it barely comes up. I think Elena is a transfer student, so she counts.
School girl status as of SF4:
Ibuki (shown in cutscenes and one of her alt costumes is a school uniform)
Elena, since she's even younger than she was in SF3
Not Sakura. She was in high school circa the Alpha series, which predates SF2. It's mentioned somewhere in the material for SSF4 that Sakura wears her old uniform because it's what she learned to fight in, and therefore she is most comfortable fighting in those clothes. Real reason is obviously it's her classic look, and people hate change, but she's seriously in college at least by SF4, and in her ending (set sometime after the events of the game) she's a teacher.
Makoto is uncertain because, while she is younger here, she's not shown going to, or worrying about school at all, and spends all her energy trying to build up the family dojo.
Chun Li wasn't ever a school girl in the games, so that's a given
Juri isn't either.
Cammy, same as Chun.
So 2, maybe 3 if Makoto if in school. There are five shotos in the game, not counting Gouken or Dan. So I guess the count is kind of close.
EDIT: Oh, wait, Makoto has an appropriately tomboyish school uniform as an alt costume that I forgot, so count her in.
