Civil liberties group threatens to sue By Monica Polanco AMERICAN-STATESMAN STAFF Wednesday, May 5, 2004 Sherrell Ingram has too much in common with her chosen high school prom guest: They're both girls. A policy at Lago Vista High School prohibits students attending the prom from buying tickets for guests or dates of the same sex. "It's supposed to be a special night that you're supposed to share with people you like to be around," said Sherrell, a 16-year-old junior. "If they're setting limits on who you can and can't bring just because of their sex, it's not going to be...