[K12OSN] how to determine bandwidth usage?

[Huck]

Could someone enlighten me as to how to gather information about
how much bandwidth is being used by a 'segment' of the network over X 
amount of time...

I have 1 main ltsp lab of 10 machines, and 3 auxilary locations with 3-5 
machines...and I would like to track bandwidth usage as well as how 
often those labs are used(i.e. someone is logged in).
Yes I know it wouldn't be completely accurate as students often "forget"
to logout...

I'm trying to get administration to move to Giga-bit throughout the 
school, and this would help a great deal.

Side-note..if it's possible to do the same thing for the MS and Mac labs??

--Huck