Some reputable group conducted an experiment a few years back in which students were shown no more than a minute or so -- if that -- of a teacher's class, and they were then asked to fill out a course evaluation form as if they had taken an entire class with her.
At the end of the semester, that teacher's actual students' evaluations of her were almost exactly the same as the ones written by students who had only a glimpse of her at work. Students, the experimenters concluded, know almost immediately if a class is going to be rewarding, challenging, boring, etc.
Research has also shown that many interviewers know within 20 to 30 seconds if they are interested in a job candidate.
So obviously, your first day in front of students is critical. On the one hand, I think you can win them back no matter the magnitude of the first-day fiasco. On the other hand, it sure would be nice not to have that uphill battle, not to be down two games to none in a 5-game series.
The Internet is lousy with sites offering suggestions of Kaganesque (and Kafkaesque, probably) first-day activities, ice-breaking games, throwing a ball at classmates whose names you already know, etc., so you don't need help with that from me.
My advice, in compressed form, is that you need to combine authenticity (you're a human being much like them) with authority (you know more because you've been learning longer). Be hospitable. Whatever you say to welcome them, they should hear "I'm glad you're finally here. I'm happy to meet you." Don't make them think that their entrance has initiated your 10-month slide into the pit of hell. Listen carefully. This may rattle them a bit, because they're not used to being listened to, but once they recover they'll be deeply grateful.
Here's my advice in less compressed form:
Some of your students on Monday will be as fired up as you are, but many others will be like little kids with their thumbs in their mouths, hiding behind their mom’s legs, staring up at you to see if it’s safe to come out. They have pretty much the same questions as the toddlers: Can I trust you? Are you mean? Are you gonna like me? They also want to know if you know what you’re doing, if you have patience, if you have a sense of humor, if you can hear them, if you speak their language, if you’re going to ask too much of them or not enough, and what role you’re going to let them play in this game.
And they want to know how much they can get away with and, oh yeah, what kind of thinking is required in this class? They’ll also want to know what percentage of their total school energy this class will require, i.e., in light of their other six classes, how much will they need to commit to your class?
Believe it or not, you answer almost all of these questions within the first five minutes. The way you look at them, the way you carry yourself, the stuff you’ve written on the board, the way you’ve defined yourself by what you’ve displayed on the classroom’s walls – students quickly synthesize these things and predict if this class is gonna suck or rock. As we noted earlier, their predictions usually come true.
So what kind of face should you wear on that inaugural day? A genuine one, a human one. Yours. Any other one will make you a false self, and there’s hardly a teenager alive who can’t spot one of those from a mile away.
Once you've finished with the required tasks of the day, try to work in something -- a "micro-teach," so to speak -- that shows just what this class is going to be like. You're saying, in effect, "This is how we think in here. This is how we converse. This is how I teach." Their response may be, "This is gonna be great and I can't wait for tomorrow," or it could be, "I should probably see my guidance counselor." Either way, you win.
I have written at greater length about the all-important first day here: http://gladly-teach.blogspot.com/2011/08/first-day-must-dos.html.