Here is a very interesting web based music notation software site. noteflight is an online tool that allows you to create music notation in usable forms such as scores or individual parts. There are several programs that exist to write music notation such as Finale and Sibelius but they are expensive and you have to install them on to a computer and get new versions as they are released.
This web based site allows you to do all of the notation elements that other programs do, however, you create and store everything online! You can easily share your music and get music that others have created as well. The basic program is FREE! Of course they do offer a more enhanced version for a fee.
I think that students would get a great deal of use out of this site for composition purposes. The students would also be able to enter and edit music of their own for performance purposes. So far the site seems easy to use offers much in the way of composition tools. I'm going to play with this a bit more and see how it works out as a usable tool for the music classroom.