Congratulations, by the way!
I have been advised that it's best to bring those pre-scheduled trips up when you are offered a job. Obviously if they said it wouldn't fly you would reschedule, but if they're hiring you they are probably interested enough to at least consider the request, and then you are not technically bringing it up in your first month on the job (you officially warned them before you even started).
I think that's easier said than done, though--it takes a lot of guts to ask for time off before you're even hired, especially in this economy!
I think the job comes first, but I'm not sure that it would hurt to ask. You could phrase it positively, saying that you were wondering if this would be a convenient time for them to have you off, since you'll still be new/learning the ropes, or if they need you to reschedule for March. (I know at some places it's actually BEST to take time off soon after you started, because you'll be too busy once you've been fully trained!) I'm assuming you know the school schedule a bit, so if things will be less busy for whatever reason in October, mention that, but I would only bring up that information to let them know that you are considering their needs as well as your own--not to press the issue and make an argument.
In other words, you're saying: "I would like to take this time off, and I thought that it would be a good time for the company/school, because ______. But if it is not a good time for me to take off, then I would be happy to do this other option instead." That shows them that you are putting your job first and even considering their needs as you ask for the time off, though it does establish that you have a life.
But I agree that if they seem to be very strict about time off it might be best not to try. If they're at all flexible, though, I'd make a go of it but leave the final decision in their hands, provided you're not going to lose money by canceling. If you will be out some money, I would press a little harder and maybe not bring up that you COULD go another time if need be.
And whatever you do, do NOT ask for the time off and then if they say "no," call in sick during the days of your vacation. I've talked to bosses whose employees have done that, and it doesn't go over well.
(Sherry E, that is terrible!

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