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The Desperate Housewives actress announced her unnamed role in the sequel to Lindsay Lohan's 2004 comedy on her Twitter page.
"So it's official.. I will be playing the lead 'plastic' in Mean Girls 2! I'm off to Atlanta at the end of the month to shoot," she said.
Walsh, who appeared in the ABC show as Ana Solis, explained that she will not appear as a regular in the upcoming seventh season.
"I will not be returning as a series regular season 7 Desperate Housewives.. But who knows I could always turn up ;) I am focusing on film! (sic)"
Meanwhile, Disney star Meaghan Martin has also revealed her role in the sequel.
The Camp Rock actress recently appeared in the cancelled ABC Family television adaptation of 10 Things I Hate About You.
Martin announced on her Facebook that she will be playing "the lead role of Jo!" who will befriend outcast Abby and take on the school's mean girls, the Plastics.
Further casting announcements have yet to be made for the film, which begins shooting in Atlanta early next month.
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