I think the chance that the person creating the "dumb" terminal is more competent at locking down the system than students are at breaking out of the "dumb" terminal is low. I work at a bank, and they have layers and layers of lockdowns for employees that take a couple of minutes to defeat with almost no skill. I doubt schools will do better.
XP has vbscript in the command line, giving you more processing power than a standard calculator... Not that people would have time to code, but maybe there could be an advantage to be had. A custom Linux build would be easier to lock down, I think.
That's what human proctors/supervisors are there for at exams.
Exams problems are designed to make use of only basic scientific calculator features, I doubt you have the need, or the time, to start sneakily writing VB scripts to try to gain an edge, while the proctors are not looking.
