Concentration is the big problem - esp without medication. The best, I have have found, is little and often, but mostly on the child's terms. If a child with ADHD doesn't want to listen or learn, they won't - they will fiddle, wriggle, sit silly on the chair, drop books / pencils, take rubbers to pieces, faff, drive you potty with silly questions, get up and walk around, be distracted by a spider on the wall, need the loo, poke each other, ball up paper and throw it, need a drink, start an argument about nothing, call out when you're busy etc etc. That's what the teacher means by 'behaviour' - but in reality, it's not bad behaviour, it's the condition.
The child can rarely help the 'behaviour' - though having said that, some of it is learned, as adhd children know how to get out of doing things and will also know exactly how to wind you up because they have practiced it on the class teacher and got it down to a fine art.
So if child A learns best after snack and a run in the playground, hit the books for 10 mins max, then have a range of learning games available. Child B, however, might come in from the playground totally hyper and need a good half hour to settle again, so provide activities which engage him until he is receptive to learning.
Most teachers struggle with adhd children in their class because they are so busy with the other children. They just cannot give the child the attention he needs to become engaged, so they get frustrated, the child thinks 'hey, I can get out of doing this if I carry on' and you get a vicious circle. For you, starting fresh, I would recommend you don't put yourself in the position where you have to argue or tell the child off - you will lose once you are at that point. Be fun and engaging and provide lots of games, change the focus regularly to stave off boredom and let each child learn at their own pace.
Plus - something I learned very early - never back an adhd child into a corner - 'do this or else' just does not work - they will go for the 'or else' every single time! Just to see if you carry it through
I'm making it sound awful aren't I ... and it's not! It's just a matter of knowing how to manage the condition. Once you've got a handle on each child and you are able to work with them to get the best out of them, you can really help them to learn... one-to-one is really valuable for adhd children and they flourish with attention.
Anything else just shout! Sarahx