I teach a lot of writing and I’ve started teaching exam prep. One of the big things on the ACT is spotting grammar errors. A friend clued me in to an fun game she’s been playing that does exactly that. In the game you are a grad student buried in debt who takes a job with an evil professor whose goal in life is to fail every student. Your job is to make sure every paper is an F. The game gives grammar tips and is timed which is useful training for standardized tests. I plan to buy the full game eventually, but you can get a lot out of the free version if you want to test it out before you sink $5 in it.

http://www.gradinggame.com/