A bit late to this.
2 things:
1. First off, before anything else, you want to make sure you've got classroom discipline in order or you'll never be able to teach anything. You basically want to start off tough, not smiling, and with high expectations. Then reward their hard work by easing up a bit as you go. Right from day one:
* Try to memorize some names ahead of time. You should have a computerized attendance system where you can see pictures of the students.
* You arrange seating on day one. You are the boss. Let them know it. You are not their friend.
* Have some simple class rules ready on day one. Go over those rules on day one.
That said, it sounds like older students, and it's an elective class, so you shouldn't have much discipline trouble (though don't bet on it). In all likelihood you can start easing up on discipline after a few days and everything will be fine --- but they must know that you'll bust them if they do anything that damages the learning environment you're trying to create.
2. Aside from discipline, here's my advice on actual teaching: At first, you want to alternate between "smart small and get fun things working" and "grand overview to give them a feel for what the heck they're doing". For example, on day one you might be doing stuff like telling them exactly what to type to make something interesting happen. Once you've shown them that they can produce neat stuff, they're going to want the overview stuff. After enough overview stuff, they're going to be like, "ok, enough already, let me do it". Then after some labs when they've moved on and are getting frustrated again, you go back into "overview" mode again. And so on.
Soon, your handholding isn't as explicit anymore, and your grand overviews start to get deeper, and you find yourself settling into the standard "lecture, work, lecture, work" style.
* * *
Regarding the split between the grade levels, I'd suggest trying to arrange things so that sometimes the more experienced students are working on their projects while you work with the less-experienced students to help them catch up. Relying on pairing less-experienced with more-experienced to have the more- teach the less- will probably lead to resentment ("I didn't sign up for this to teach! I signed up to code!").