Gabrielle Union, a regular on “7th Heaven,” has signed to co-star in “She’s All That” opposite Anna Paquin and Freddie Prinze Jr. for Miramax Films and Tapestry Films.Union will play a member of the “Popular Girls” clique in the high school romantic comedy about a teenager who bets he can turn the school’s geekiest girl into a prom queen and then falls in love with her. Union also joins Rachel Leigh Cook, Matt Lillard and Elden Hensen. Pic begins shooting this month in L.A. under Robert Iscove’s helm. Lee Fleming scripted. Peter Abrams, Richard N. Gladstein and Robert L. Levy are producers. Jeremy Kramer is an exec producer.