Brian often interrupts his teacher while she is speaking and frequently forgets to complete his homework assignments. he also has difficulty taking turns in playground games with classmates. brian most clearly exhibits symptoms of

ADHD (attention deficit hyperactivity disorder)

Explanation:

ADHD is a neurodevelopmental disorder in which a person often acts inappropriately with regards to his age, has difficulty paying attention to things and often acts without taking into consideration the consequences of his/her actions. A person suffering from this syndrome exhibits symptoms that are often noticed in the school, home or during recreational activities such as playground games and mostly before 12 years of age. The child also shows restlessnesss and hyperactive behavior. Since Brain exhibits all these symptoms - hyperactivity (interrupts the teacher), forgets homework (lack of attention) and difficulty taking turns in playground games, he is most probably suffering from ADHD.