ACH, it also depends on your LEA. Some are very good about statementing, whilst others are awful. You have to have alot of evidence, involvement from agencies outside the school eg: behaviour clinic, paediatrician, Ed Psych, to get a statement granted. This can take a long time, especially as these other people are so stretched. Is there anyone locally that you know that you could ask, someone who might be going through the statementing process, for any reason?
The thing about statements is that once your child has one, then the LEA has a legal obligation to provide the support that is written in the statement, and some LEA's baulk at this on grounds of cost. So you may have a fight on your hands. I think Myalias is right, it is often hard to get a statement on behavioural grounds alone - they often work on the premiss that because a child is bright they don't need the extra help. Interestingly, my LEA doesn't include bright children on the Special Needs Audit, although they do provide training for the 'Able child'. The person who did my school's training day admitted that bright children have as many special needs as the ones who are academically poor!
If your child has been diagnosed with ADHD, then the school should have copies of the relevant reports and your child should have an IEP. If they are being difficult, ask to speak to the SENCO - Special Needs Co-ordinator. If you get no joy there, go to the head, and if you still feel that they are being difficult, then maybe looking for a more sympathetic school is the answer.