Sixth graders are cool, you're lucky. Although, I'm confused, do the sixth graders in your area go to different subjects? In my sixth grade it was still all one class, with sojourns to other rooms for reading and math. Anyway, sixth graders are just starting to get sassy, depending on your neighborhood. But you may still be able to inspire them with the love of learning.
Um, yeah, I have a horror story:
Statistics (math! math I've never seen before! A whole different kind!)
Speech (I've been putting this one off since age 18! God knows why I can do everything else in front of people but speak my own words. I'm developing flop sweat just thinking about it.)
Torts
Law Office Management and Procedures
Legal Aspects of Evidence
That's my semester, as of now. Probably not what you meant.
I was trying to think of first day horror stories from my friend who is a teacher, but couldn't. She has lots of other horror stories, but they didn't happen on the first day.
Oh, how bout this? One day I asked my sixth grade teacher about gaining weight, and why she had gained weight, what the whole process was, etc. I was concerned about my weight gain (minimal) and she was fat but used to be athletic. So, I thought she would be an expert. Plus, I hated her and she was mean to me.
How's that? Is that good?
You'll be fine, everyone will love you, sixth grade will be interesting.

That would be one of my few grades of choice, honestly. I think junior high would be harrowing, and highschool dispiriting, and younger elementary boring.