I found a special deal that might be of interest to you.
I scoured the web, and found the best deal I saw was on Bestfares.com--they usually do have the best deals, anyway--I got tickets for my upcoming trip to Toronto for $175 when the fare on AC was $330...*and* it's open-jaw!
In any case, here's the deal: It's what they call a "Snooze, You Lose" special...it's a really low fare, but it only lasts for a limited period of time. This deal may or may not have expired by now--there's no expiration date on the offer, but sometimes they do without anyone telling you.
Anyway, it's a fare for $156 R/T from Dallas to NYC (airports not specified). There is a minimum stay of one night, and the ticket must be purchased 7 days in advance of travel.
To check this out, what you'll want to do is call (800) 645-7829 between 9am and 6pm CST. When they ask you what story number you're looking for, tell them 10079955.
Also, just as general information, depending on what airline you fly on, they might charge you a $20-30 fee for handling an unaccompanied minor. It really depends on whether the itinerary involves a change of planes...no change of planes, no fee.
I checked NW's flight schedule, and they do not have any continuing or nonstop flights to any of the NYC airports. All connecting flights change in either Detroit or Memphis. You'll have to call them directly about whether they charge for unaccompanied minors or not, because it doesn't say on their website (argh...what an exasperating site...AirTran has that information readily available on their site).
Hope this helps you out...