I bought a $5 kitchen timer from Target (Hamilton Beach, I think).I then opened it up (easy with a wee little screwdriver), and ripped out the noise-maker.Presto! Silent timer!
use a stopwatch, and open it up and take out the speaker plate. its extremely easy, and if you feel like you'll screw it up then buy 2 and experiment the first time. that's what i did but ended up returning the second one because the process was so simple. i think that's your best bet.
how can i prove to the proctors that it makes no noise if they give me crap and say no stop watches?