Is this for a BA, MA or PHD level thesis? Only reason I ask is it might matter what degree of complexity is required. Perhaps the following might be acceptable:
First off, it would seem to me that it would be reasonable to require that ALL cell phones be turned
off for an exam.
That being the case, then you could design a receiver system to detect just the
presence of a cell phone signal.
An array (or perhaps a
single) antenna [stub, parabola, whatever]), might be strategically placed in the class room at, say, the center of the ceiling facing down. It (and its receiver system) could be de-tuned (and isolated) to limilt its range, and yet capture
any cell phone emmissions.
That would be a sufficient indication of possibly nefarious use and would avoid any accusations of "spying" on a student's personal comms (which are being spyed on anyway, just not by you
).
<EDIT> Of course, YOUR cell would have to be off as well...
Then again, the idea might require a Faraday cage around the whole classroom, which might make the idea ridiculous. Be worth a shot, though, I'd think. Signal strength IS a factor of distance from the source.<EDIT/>