What about the well-behaved difficult to diagnose child?

These are the ones who have AD/HD of the predominantly inattentive type, most commonly girls (but many boys fall under this category as well). These kids present with the cognitive impairment of AD/HD. They may do relatively well until schoolwork becomes more difficult. Some may present with academic difficulties only in Junior High, when the work becomes more complicated and the requirements complex with different teachers and the need to change classes. The impairment may begin later in life, some symptoms however must be present before 6 years of age (see DSM IV criteria).