It may be a bit more work, but what I did was take Latin classes to prepare for the SAT (a college entrance exam for US students) as a majority of more complicated words are based off of it. We all hated the SATs, but I actually learned more about grammar and vocabulary in my SAT prep classes than in my English classes. It has been years since my SAT days, but I recall you can try a few sample questions on the College Board website.