At heart, Gaggle is a semi-free filtered email solution for students. Free being in the sense that the unpaid version includes banners/adds and really slows down a network. However, after using the purchased version of Gaggle in our district for the last few months, I can say Gaggle is so much more then just an eMail system.
Gaggle has a variety of components that makes it appealing for education such as: blogging, discussion boards, chatting, digital lockers, and email. Since all of this is "housed" in Gaggle it is all filtered.
Another nice feature is the admin user interface which lets you tweak settings to your hearts desire. You can set it to email yourself for blog comments, blocked messages, etc etc etc. Like any filtering system you can set it to as high/little as you like and include any keywords that might not be in the system.
Finally, one other great selling point is that it works w/ Google Apps. You can have Gaggle "house" a GMail account which is great if you like to par take in the Google universe or Googleverse. For more info on Gaggle/Google interaction click here.
If you are strictly looking for a blogging solution for education which is free, take a look at Edmodo. Edmodo is a microblogging solution for education which is very user friendly; think Twitter for education.