There are all sorts of strategies which can be used to help children with ADHD improve their focus. Ensuring comfortable clothing and giving them movement breaks, for example. Visual aids, breaking down tasks, positive reinforcement (children with ADHD not only receive more negative feedback than NT children but they also have heightened sensitivity to it). Plenty of playtime outside, even at secondary level. Sufficient sensory stimulus, but also balanced against the potential for sensory overload.
Part of the problem is the classroom environment in many secondary schools. It lacks the kind of sensory input that helps children with ADHD to regulate. Schools are increasingly built like office blocks or conference centres - too clinical, insufficiently ventilated, strip lighting etc. They don't offer the kind of sensory input that children with ADHD often need to regulate. When I was at school, the main school building was a dark, drafty, old-fashioned building, lots of the class-rooms were wood-panelled, the windows were always open. Half the classes were held in drafty, freezing cold portacabins, so we almost always had a brisk 5 minute walk outside in the cold between lessons. Grass everywhere and kids would eat lunch outside in the summer months under the trees. It sounds dreadful, but it offered so much more sensory input than many secondary schools nowadays.
I think that secondary school design plays an underacknowledged part in the difficulties faced by ND children, alongside zero-tolerance policies. They are spending their days in environments which just don't contain what they need to thrive.