If you're on a quest for good SF for YA readers, I highly recommend Time Shifters by Shanna Lauffey. While it's not targeted to YA, YA readers seem to really enjoy it.

Yes, after your earlier recommendation I did try the first of the Time Shifters, and did enjoy it. The one disappointment was the length, which fell short of full novel length. I felt I'd got a snack, rather than the meal I'd hoped for.