Police are searching for a teenager who they say assaulted an MTA bus driver while he was on the job in The Bronx. According to authorities, the suspect boarded a Bx21 bus back on Aug. 15, refused to pay and punched the operator in the face. They say the teen then fled down Westchester Avenue. Riders say they are unsurprised by the incident and that the MTA should improve security measures for all bus drivers. An agency spokesperson says only some buses are equipped with a safety glass shield. The bus driver, who spoke off-camera, says he always fears non-paying customers. Investigators have described the suspect, who was caught on surveillance video, as Hispanic and between 15 and 18 years old. Anyone with information is urged to call Crime Stoppers at 1-800-577-TIPS.