[mesmark]

Well... I can tell you what I think is the most difficult.

HS students can learn and remember a lot more and in a shorter time than K students. The bad part is they don't want to practice and since they 'understand' the language, they want to move on. (quite different from teaching K.)

To battle that aspect, I generally try to supply new vocabulary to any unit or area that I want to review or practice further.

They still want to have fun and they can be great classes. You can play more complex thinking games and still have a good time.

Good luck.