To bake a decent brioche in 6.5 hours is doable. You have to make sure the area that you are resting it is at a good temperature......usually a warm, draft-free spot to get a good fast....yet not too fast rise. I make viennoiserie product everyday as that is my speciality so I usually make sure I get the overnight in the fridge part in for full flavour development however, it is not necessary in an exam that is just ensuring you know how to make brioche.
The sequence of brioche: mix the dough; place in a clean bowl, cover loosely with wrap and rest in warm, draft-free spot until doubled in size (approx. 1 hour in a warm environment); de-gas, re-tuck, place back in bowl, cover tightly with wrap and let it rise again in a warm spot until doubled in size (again, approx. 1 hour); ready to turn out and form, cover forms loosely with wrap; let rise in forms until about doubled (the dough will spring back when poked lightly with your finger); time for bake.
This should take you from beginning to end approx. 3.5 hours if you are a newbie and taking your time. The hard part is finding a warm, draft-free area for the rise so that your timing is not screwed up so you might want to scout that out first before exam time. If the dough rises too fast your screwed and too slow your screwed for your exam so just get the rise time down. I would practice if I were you.....don't leave it to chance but that is just me. Take your timers with you to remind you to check on your dough. This should be started first so that you are not scrambling, as tarts, choux and entrements are easier to work around.
Good luck, I wish you all the best, take a deep breath and then have fun!!