Montessori schools traditionally don't focus on memorization. You may just need to do flashcards daily and buy a Math Minute book to try timed math tests. Our son doesn't have ADHD but has a documented processing learning disability. He has extended time for tests but has never used. Doing flashcards every day for 6 months has helped alot, plus doing some Kumon workbooks. It could just be practice, practice, practice.