There are plenty of online diagramming applications, but Lovely Charts is one of the better ones to come along in a long time.
The first thing you'll notice after signing up (free, but paid to unlock everything) is the polished look/feel to it. This is one of the nicer looking online applications. Also, it is very user friendly and simple to use. Anybody who has had experience w/ Inspiration will be able to pick this up in no time.
One of my favorite features of Lovely Charts are all the different styles of diagrams you have to choose from: Flowcharts, Sitemaps, Network Diagrams, Wireframes, etc. Plus, it's very easy to save/download. I highly recommend checking this out and see no reason why it can't be used in education. The paid version unlocks all the web 2.0/collaboration features.
For more info click here.
Below is an example of a Home Network Diagram...