Here's the best place online to research ADHD (imho)
www.additudemag.com
Montessori is a GREAT place for ADHD kids (particularly adhd-c and adhd-i), and they often show zero to no "problems" whatsoever. It's because of the way the education system is modeled. They work on what is interesting to them and are ALLOWED to actually finish what they're working on (they can do maps every day for weeks if they're really fascinated with maps, or math, or reading, or, or, or) and they're able to choose works that are interesting to them in EVERY subject.
Not so, standard k12 schooling... where everyone has to learn the same things, and drop what they're doing (and not be allowed to come back to it) and science will be about ponds, or biology (even if you have a science kid who loves SPACE and chemistry) so they start hating entire branches of education. Sitting (and sitting in a certain way!) is required, so they aren't able to self regulate by standing or swinging their legs, or curling up.
The modern K12 schooling system could not be WORSE designed for ADHD kids. Montessori is one of the BEST (so are most gifted schools).
DO look up ADHD, but also realize that certain environments bring out the BEST in the disorder, and some bring out the WORST. Montessori brings out the best by and large, while public k12 the worst. One size fits all, just DOESN'T.
Do also look into other schooling options. Our area has 3 k12 montessori schools. 1 private and 2 public. The public options only have 40 spots and over 1000 kids ADDED to the 'waiting list' every year. There's also one gifted school. Both the private options are waaaay out of our pricerange (15k per year) but there might be more accessible options in your area.
We (after an increasingly disastrous first year in public school) decided to try homeschooling before making the major sacrifices it would take to pay for private montessori or gifted school. That was 4 years ago.