A prosecutor says a Tennessee school bus driver was on his cellphone and driving 20 mph over the speed limit when he caused the crash that killed six elementary school students in Chattanooga. Johnthony Walker is being tried on 34 charges, including vehicular homicide. Prosecutor Crystle Carrion said in opening statements yesterday that Walker was driving 50 mph when his bus ran off the curvy road, overturned and hit a tree. Walker’s defense says he swerved to avoid another vehicle. Authorities say Johnthony Walker was speeding when the bus crashed with 37 children on board. He was charged with 34 offenses, including vehicular homicide. A prosecutor says Walker took a cell phone call at the time of the crash. Hamilton County District Attorney General Neal Pinkston said Walker received the call at 3:17 p.m. It lasted nearly four minutes. Pinkston said the first 911 call about the crash came in at 3:20 p.m. Walker has been out of jail on bond and had to wear a GPS ankle monitor. He was also not allowed to drive.