One of the things I noticed early on is that in the 6th Grade, 9-week course of ukulele I taught, students would get interested enough to purchase their own instrument, and play on their own! But, after a while, it would join the other stuff in the closet gathering dust. Why? They had no one to play with!
It's the rare students who will continue to play on their own for its own sake. They are far more likely to continue to develop their skills if they have other people to play with, thus the ukulele orchestra is born. (Or the fretted orchestra, in my case.) This isn't a common ensemble, though. As a result, I've had to search far and wide to find sheet music that I could use for a group like this, as well as having to make my own arrangements.
My geekery is your gain, however. I've included a list of Ukulele Ensemble music that is available below, all in one place. My hope is that this list will be comprehensive. However, if you come across others, please let me know!
James Hill - Ukulele in the Classroom Series
I think we can all agree that James Hill is awesome. (Okay, I'll admit it...I envy his hair. And the fact that he has any at all...)
He is also a big proponent of music education in general, and the ukulele as a classroom instrument specifically. Luckily, he's done some arrangements for ukulele orchestra that are very approachable, and one of them is even free! .
Make sure to check out "This Little Light of Mine." It's a great starter piece for a group, and available for download free! The other arrangements are great, too, and of varying difficulty. They also include MP3 files of each part.
NOTE: If ordering directly from the website, be sure that you are ordering "C6 Tuning," unless you have adopted the Canadian D6 tuning in your classroom
These are available, though are of limited use if you have a beginning group. The easiest of them are in the Christmas books, but if your ensemble has a number of students who had already played guitar and read music, or are ready for a challenge, these might be approachable.
There are some pretty great arrangements on this site for both solo ukulele and ensembles, and you can purchase these at an extremely reasonable price. These are mostly classical arrangements, and most are in the early-intermediate category.