Two U.S. athletes, who represent the U.S. Olympic Education Center (USOEC) University resident team at Northern Michigan Univ., claimed medals at the Petrov International of Greco-Roman wrestling at Sofia, Bulgaria, March 12-13.    Claiming a silver medal at 96 kg/211.5 lbs. was Adam Wheeler (Lancaster, Calif), while Harry Lester (Akron, Ohio) claimed a bronze medal at 66 kg/145.5 lbs.    Wheeler was defeated in the gold-medal finals by Vladislav Metodiev of Bulgaria. He won three straight matches to qualify for the championship match. Wheeler opened with a win over USOEC teammate Adam Haake (Winona, Minn.), He defeated Giorgious Koutsioubas of Greece in the quarterfinals and Stanislav Rodionoc of Russia in the semifinals.    Lester won three straight matches to start the tournament, defeating Ion Pansit of Romania, Mukhuran Matchuradze of Georgia and Vasil Teodossiev of Bulgaria. In the semifinals, he was defeated by Emilian Todorov of Bulgaria.    In the bronze-medal match, Lester stopped Ivan Aleksandrov of Israel, to win one of the two bronze medals in the weight class.    Six other U.S. wrestlers placed in the top 10 of their divisions. They were  * Andy Bisek, 10th at 74 kg/163 lbs.  * Tim Bleau, seventh at 74 kg/163 lbs.  * Cheney Haight, ninth at 74 kg/163 lbs.  * Jeff Courtney, 10th at 96 kg/211.5 lbs.  * Gerard Habibian, seventh at 96 kg/211.5 lbs.  * Joe Wood, 8th at 120 kg/264.5 lbs.    The U.S. team was coached by USOEC Greco-Roman head coach Ivan Ivanov. The USOEC team spent several days in Bulgaria, in joint training overseas. Ivanov is a past World medalist from Bulgaria and helped organize this training and competition tour.