I agree with the suggestion of considering PIano Town. If you want to see what the pages look like, visit Piano Town on facebook
. They have done page-by-page images for the lesson books; it started in May 2011 with the primer book, as of today (6/28/12) the newest entry is Level 4, page 4. Navigating through is a little tricky with facebook timeline, but you CAN see everything, and observe teacher comments.
* Let the other 12yo use PA Accelerated.
* Let the 8yo use the regular PA course. If he really flies through it, you can supplement or skip some pages.
* Begin one of the 12-year-olds in a faster-paced book like Alfred Basic Older Beginner, then change to PIano Town or Alfred Premier later on.
* Use Hal Leonard Adult method for one of the 12yo students; it has less of the "older pop music" Trista mentioned. :-) I think theory & lesson & technique are all integrated into one book.
* I have heard good things about Noona books, though I have not tried it myself; there are some reviews of this curriculum and many others on the Yahoo Groups piano teacher forum. You cannot access the information unless you are a member, so visit the site and ask to be a member. :-) This is a link to the Yahoo Piano Teachers Group
Once you're in the group, visit "Files", then "Kevin C Course Reviews". Kevin recommends Noona Clavier for teenage beginners.
I look forward to hearing about your solution for choosing books for these students. All the best to you!