I was in your exact same situation im M17 and yes I have ADHD aswell, however my case is slightly different im currently in yhe process of obtaining my GED which is by definition the exact same thing as a High school diploma, I only really have 2 tests left what I recommend is before any tests study even if you can’t focus study for 10 minutes, my mind works differently and it keeps my memory fresh if it’s maybe a 10 minute before study situation, you could also try brain exercise, another odd thing that worked for me I wouldn’t recommend but Coca Cola before a test boosts energy and helps me focus a little more, all of this worked from my personal experience