Given the choice, I highly recommend splitting the class into levels as opposed to mixed abilities. Some like the idea of sitting a higher student next to a lower student but it's a lot easier to plan and produce a valid lesson for students of a similar ability and hence keep their attention for longer.
Some great ideas here, I've filled a page of notes already! I'm of two minds as to whether I should stick with just an introduction for my first lesson and save the rules for the second? I like the idea of spending some time on them and having the class help to choose them, but I'm a stickler for discipline and don't want to be giving out to kids for stuff in the first lesson if we haven't even covered set rules yet. I'm wary of High School kids thinking they're above playing games/activities, etc, but if sleep is high on their priorities, I'm thinking I should try and keep them on their feet. What's the general results of this from others, if you've tried?
How long should the introduction ppt last for? I want the students to know a bit about me to ease any tension but obviously don't want to make it really long and boring!