A Righetti school bus driver and three students were injured yesterday when their bus collided with a farm truck at 7 a.m. on Foxen Canyon Road near Siquoc east of Santa Maria. Apparently the spreader fertilizer arm on the truck struck the bus as the two were passing by each other. Forty-four-year-old Yolanda Zepeda was driving the bus when the collision occurred. She lost consciousness and the bus drifted off the side of the road hitting a fence and a tree. Ten kids were on the bus at the time. It took firefighters an hour to extricate Zepeda. She was taken to the hospital with major injuries. Twenty-five-year-old Hugo Medina who was driving the truck was not arrested. The CHP is still investigating.