USA Student Travel handles all of the booking/tickets for the YES program... Here is the link to the Disneyland individual enrollment program:
http://www.usastudenttravel.com/disney-yes-programs-dl/
From here, you can see all of the 2012/2013 class dates, as well as access the individual enrollment forms. Just fill out the form and fax or e-mail it per the instructions. If they don't contact you in 3 business days, there is a number on the form you can call (I ended up having to call). Once they confirm that there is room in the class you want & how many tickets you need, they will e-mail you an invoice. You are then able to make payments online through the USA Student Travel website.
By the way, I actually had the class cancellation info in an e-mail... Here is the USA Student Travel rep's quote: "Payment is due about 4 weeks before the start date of the program. If you pay now and need to cancel or the class is cancelled you
can choose another date/ class, you can be refunded, or since you would have prepaid for your tickets you can use your tickets but it would be without the Youth Education Series."