( CNN ) -- A tractor-trailer slammed into a school bus driven by the mayor of a Louisiana town Friday morning , critically injuring two children . Twenty-one children -- ranging in age from 5 to 17 -- were on the St. Landry Parish school bus when it was hit while pulling back onto a highway in central Louisiana , Louisiana State Police said in a news release . Earlier Friday state police indicated that the crash left three children -- ages 5 and 15 , plus one more whose age was unknown -- in critical condition . As of 5 p.m. , Port Barre High School Principal William Duplechain said , 19 students had been treated and released from hospitals , and two remained in critical condition . They included a kindergarten student , who was unresponsive , according to the principal . The other was a middle-schooler who was placed in a medically induced coma so surgeons could operate on his broken leg . Gil Savoy , a substitute bus driver , was driving the students when the accident occurred , Port Barre Town Clerk Toria Comeaux said . Savoy is the mayor of Port Barre , a community of about 2,400 people some 30 miles north of Lafayette . He had pulled over on the side of U.S. 190 and , after all the students had boarded , began to slowly move forward `` with all of his warning devices still activated , '' according to Louisiana State Police . At that point , an 18-wheel truck driven by David Cox , 52 , of Dallas , Texas , rammed into the bus from behind , police said . Comeaux said that the truck , which she was driving behind , had debris coming off it at the time of the crash . Both Savoy and Cox suffered minor injuries , with `` routine toxicology samples '' taken from both , the state police said . No one has been charged , and the investigation is ongoing . Video from CNN affiliates KLFY and KATC showed severe damage to the back of the bus and to the tractor-trailer 's front . Anthony Standberry , a St. Landry Parish school board member , told KLFY that the speed limit on that stretch of U.S. 190 is 65 mph . He said he favors working with parents and school officials to remove all bus stops along major highways . CNN 's Max Newfield and Vivian Kuo contributed to this report .
UP : 21 children were on a school bus when it was hit from behind them . NEW : The bus , driven by the town 's mayor , had picked up students alongside a highway . At least two students are critically injured .