The pilot season was originally voiced by Amber Hood and the rest are Grey DeLisle-Griffin. She's done voices for most of the male and female characters for episodes at one time or another though she's mainly the voice of Vicki and Tootie.
(Vicky, Tootie, Veronica, Chad, Principal Waxelplax, Tooth Fairy, Happy Peppy, Betty, Molly, Tom Sawyer)
Her IMDB is below and it shows her huge list of shows she lends her voice talents to.
www.imdb.com/name/nm0217221/