Three things:
1. Clear parental expectations, follow-through and healthy routine at home.
2. Medication for ADHD overseen by a psychiatrist.
3. If they are eligible, placement in special programs/pull-outs/aides for school. I hope they both have IEPs and their accommodations and services are up to date.
Here is my coaching experience:
My son with ADHD and daughter without ADHD have been coached and tutored by me since preschool. It's a very close relationship that needs to develop over time and requires great trust and compassion on all sides, and above all, respect.
You mustn't be afraid of pushing the limits of what you can all endure, to define where the boundaries are. We have yelled and screamed at each other. We have worked for hours straight. We have cried. We have felt a great sense of accomplishment and joy. Usually we don't do any of these things and just work quietly. We have a very close bond.
At first I just wanted to practice what my son's therapists had given us as homework (he needed intensive PT, OT and speech as a toddler), but then I realized he learned so much better on a one-on-one setting that it became a mini-homeschool, with music, math, literature and our native language. I extended that model to my daughter.
During DS's 5th grade year, I worked to have him placed in a special program for middle school. This meant a round of neuropsychogical evaluations early on and spending the rest of the year persuading the relevant people in MCPS as well as the IEP team that he needed that placement.
He is doing much better now, and I am much less hands-on. But this is possible only because we have worked so long together, that he knows how to study, what my expectations are, and that I trust him (most of the time, and not with a book!). His sister just follows along - it's so indecently easy when there's no ADHD
I believe any parent can teach study skills and the elementary and middle school curriculum. Inattention, processing speed and focus can be helped by medication. However, if there is too much defiance despite your efforts to build trust and respect, you probably need to work with a therapist and/or discuss with a psychologist.