A Milwaukee Journal Sentinel report says attorney Anthony Cotton made the argument in a motion Thursday. He says the exam would violate her right to remain silent.
Cotton represents one of the two girls accused of stabbing another 12-year-old in May and leaving her for dead.
During a hearing Wednesday, prosecutors asked that the girl be given a mental exam to determine whether she had any mental defect at the time of the crime. The judge granted the hearing over Cotton's objection.
Cotton says the results of the exam would presumably be turned over to prosecutors, violating the girl's right to remain silent.