What's wrong with Facebook?
I'm not a big Facebook fan, but most of the kids and their families are probably already on Facebook. You can easily setup a free public Facebook page. You can specify the URL as something like www.facebook.com/YourHighSchoolEtc
. You can put your pictures in separate albums and as far as I know, there is no limit to the number of pictures.
Since most of the kids already have Facebook it's easy for them to share, comment and like your pictures.
You could also do this with Google+, but due to lack of popularity, you will probably have a smaller audience and it's less convenient for the kids.