A 13 year old schoolgirl claims she was groped yesterday by a man in a Midwood subway station.  She told The Post she was on her way to school at around 7:30 a.m. when she was approached from behind by the man on the Brighton-bound Q train platform at the Avenue H station.  She said the man pursued her and asked, Why don't you white people want to talk to black people?  She said she went through a turnstile to leave the station, but the man suddenly fondled her and fled.  The attacker was described as a black man in his 30s, 5'7 and 160 lbs.  He was last seen wearing a red plaid shirt and blue jeans.