After last week’s Virginia Tech massacre, hundreds of colleges are considering a text-message emergency-alert system, and thousands of students have signed up for the cellphone service on campuses where it’s already in use. “The standard changes after Virginia Tech,” said Mark Rosenberg, chancellor of Florida’s state university system. He is asking the state Legislature for $1.5 million for new emergency-alert systems that include text-message notices for the 11 state-run campuses. Virginia Tech school officials did not alert students until more than two hours after the first 911 call reported a shooting at West Ambler Johnston Hall. Most schools including Virginia Tech warn students with e-mail, but that may not be read for hours.