The LA I worked for adopted exactly the same model you describe. Are you connected directly to an RBC or are you connected to an RBC through the LAs network ??
The only reason I ask is because if you are connected via the LA's network then the reason they charge more the bigger school you are is because they argue that the more kids you have then the more traffice you will create across the LA's network, so therefore they pretend that they need to invest in more infrastructure... Which is total rubbish, but they think they can charge what they want, and its only a matter of time before every school has an independent internet connection....