[interbaoui]

When I was in university here in Canada, I simply asked the invigilator (who usually happened to be my prof) if I could go outside the room and pray and I would be allowed to do so. People are permitted to go to the bathroom so why would they stop you from going outside and praying? Just find a stairwell or any area away from prying eyes and pray. Try to pray near the exam hall and don't wander off because that may make the invigilators suspicious. Pray the fardh and return to the room. It should take less than 3-4 minutes.

Come prepared with wudhu, of course.

The worst exam times are in the winter, because during the duration of the exam, two salah times can come and go (i.e. 'Asr and Maghrib times, especially for us Hanafis) and some people are either too fearful and make up excuses in their mind to not pray. We should never hesitate when it comes to our deen. We should realize that other people are able to go to the bathrooms during the exam and usually take about 5 minutes. Our salah would take even less than that so why should we worry?

Another suggestion is to wait until you're confident that anyone who is going to write the exam is already in the room (about 45 mins to an hour after the exam has started) and then ask to be excused for prayer. This way, you won't be distracted by others coming in and the invigilators won't be suspicious of you passing on information about the exam to latecomers.