To answer your questions...
Free time:
It varies, but during an average week, I usually have about 2 hours or so of free time. I use the 2 hours to lesson plan for the most part. Midterms and Finals week are awesome though, and I get a ton of free time during the exams.
Workload:
I'd say, higher than most elementary school teachers, but not as much as a Hagwon teacher. I teach 22 hours a week, and make 5-6 lesson plans a week. My Middle school does levels for 1st and 3rd grade. So I make a lesson plan for 1A, 1B, 3A, 3C, and 2nd grade classes.
Education clown/Actual teaching:
I get to teach the Listen and Talk section of my textbook, and since that is only about 3 or 4 pages, I always supplement it with my own activities (role-plays, bomb games, running dictation, etc.). I definitely feel like a teacher. My co-teachers are helpful with discipline, and the occasional translating (for lower-level classes), but I would say that I'm in charge of the class. All lesson planning is done by me, and I make the presentations (or modify/take from Waygook...hehe) myself. My co-teachers will help me run activities, but for the most part, they either sit down and do their own work, or run in and out of class every 10 minutes or so (can be a little frustrating at times).
Main difference that you will probably get at Middle School is behaviour. Students at my school behave well for the most part, but I taught at a summer camp for elementary school and the difference was noticeable. Elementary school kids are way better behaved/likely to settle down than Middle school kids are. On Fridays, I teach 5 classes, all-boys, 35-40 students, and they can be a handful. I love their enthusiasm, but the noise levels can get out of hand sometimes. Basically, when they're into your lesson, it's awesome. When they could care less? It's pretty awful. I also feel like I can relate more to my kids than I would with the younger ones at an Elementary school.
Anyway, that's my two cents worth. As you know, it's Korea, so your experience will all depend on a roll of the dice pretty much. Good luck!