Check out the links to all the other lists of difficult words. Frankly, there's not much different, at this level, between a 10th grade word and a 12th grade word, for example.  Also, encourage students to find their own difficult spelling words and contribute them to a classroom list. They will likely encounter more words in their reading and subject matter texts. When they are exposed to words that are relevant to them, they are most likely to be motivated to really learn and use them!