don't really have much of an idea but maybe the school when they wiped (or re-imaged) the netbook they removed something that was needed to "create" the additional account. You probably need to speak to the primary school to see what the go is and check with regards to how the "contract" works with regard to you "owning" the netbook. I know that here in some SA schools, students are able to purchase their old laptop at the end of Yr 12 but there would be some things that would be wiped from the laptop at the end of the school ownership of the laptop due to licensing and security things (can't think of the right term).
You may need to eventually (after the 3yr contract expires) go to another IT tech to get some of the settings re-set so you can use it for the net at home.