Try your very best to catch the child being good. Many of their responses are impulsive. We tend to notice the socially inappropriate responses and miss the many acts of generosity and apparent maturity that may also be the impulsive response. Teach the child how to draw up mind maps. Encourage the use of this technique in lessons, it will give the child a greater sense of being in control of what is going on.
Many unfocused children have a tendency toward being visual learners.
Therefore some form of visual cue allied with your verbal explanation will probably aid comprehension of the task being set and the expectations you have for the piece of work being set. Keep every thing as simple as possible. Make things fun so that they will attract the child's attention thus increasing the chances that the message will be absorbed. Use difficult situations in the child's day as opportunities to teach the child and give feedback. Make your and the schools expectations very clear.