My daughter has off-the-charts ADHD and anxiety and is struggling in school. I did buy her a fidget spinner and fidget cube but she also keeps a stress ball in her book bag. I bought these tools more for her anxiety than her ADHD though because she is a picker from her anxiety and needs something to keep her hands busy. From an ADHD standpoint, I’m not sure that these help a lot but from an anxiety standpoint, they do seem to help.
And thank you to ADHDmomma for pointing out that kids with ADHD shouldn’t be told to try harder. They actually are trying but they’re working against their own DNA make-up.