By Dianne Solis The Dallas Morning News/TNS DALLAS - Greisa Martínez Rosas was still a teen when she led a 2006 walkout at her Dallas high school over proposals that threatened deportation for her undocumented family. Today, at 32, from Washington, D.C., she leads the nation’s largest organization for undocumented immigrants who call themselves Dreamers. As the head of United We Dream, she’s front and center of a defense to keep alive the 2012 initiative known as Deferred Action for Childhood Arrivals, or DACA, that provides work permits and deportation reprieves. A White House exit for President Donald Trump, a DACA opponent, is only weeks away, but DACA still faces a huge challenge: On Dec. 22, a federal judge is scheduled to hold a hearing on its legality in a case brought by the State of Texas.