FITCHBURG -- A former Fitchburg charter school sex-education teacher pleaded guilty Friday in Worcester Superior Court to a charge she raped a 14-year-old student, according to a report. Rachelle Gendron, 28, reached a plea agreement and was sentenced to 2 1/2 years of prison time and 10 years of probation, according to Tim Connolly, spokesman for Worcester County District Attorney Joseph D. Early Jr.