A student, a teacher and a security officer at a District of Columbia elementary school were seriously injured yesterday when a stolen car jumped a curb and struck them as they were leaving school. The student suffered a fractured thigh in the incident, which occurred outside Whittier Elementary Schools at Sheridan and Fifth Streets, NW, according to Mafara Hobson, a school system spokeswoman. Both of the teacher’s legs were fractured and the security officer was cut in the incident, Hobson said. None of the injuries was believed to be life-threatening, authorities said. Police said four youths were apparently in the car when it jumped the curb. A 17-year-old youth was taken into custody and three others were being sought.