Thousands of protesters are marching from Madison Park Technical Vocational High School in Roxbury to the Common today in a student-led march to end gun violence.The March For Our Lives, which will coincide with dozens of other demonstrations nationwide, is in response to a horrific school shooting in Parkland, Fla., last month that claimed the lives of 17 people.“We’re not backing off until there’s zero gun deaths,” said Vikiana Petit-Homme, a student at Boston Latin Academy and one of the organizers of the march.