Assuming that "Xiaoming won the white gold medal" is correct, then "Xiaohua can't win the gold medal" is also correct, which is inconsistent with the condition that the teacher only guessed one right, so the assumption is wrong. Xiaoming actually didn't win the gold medal.

If "Xiaohua can't win the gold medal" is correct, then according to "the teacher only guessed one", it is inferred that Xiaoming didn't win the gold medal and Xiao Qiang won the bronze medal. So no one won the gold medal, so the assumption is wrong. It should be Xiaohua who won the gold medal.

The first two assumptions are all wrong, so "Xiao Qiang can't win the bronze medal" is the one that the teacher guessed right. Then the other two sentences are wrong. Xiaoming didn't win the gold medal, and Xiaohua won the gold medal. And because Xiao Qiang is not a bronze medal, Xiao Qiang can only get a silver medal and Xiaoming a bronze medal.