TWO teenagers tied up the staff of a 7-Eleven shop and wore their uniforms while they ransacked the store. Chan Kwok-kin, 16, and Wu Kwok-wai, 16, pleaded guilty to robbery in the District Court yesterday. The two teenagers, along with accomplice Wong Nga-lun, 17, burst into the store in Cheung Sha Wan, in the early hours of February 19, 1995, and threatened the two staff members with a paper cutter. Woo kept guard over the captives while Chan and Wong, both dressed as 7-Eleven workers robbed the store of cigarettes, comic books and cash, totalling more than $15,000. They later admitted a string of similar 7-Eleven robberies.