Even though "luck" is often used a noun with a specific referent of
a supernatural force that causes the dice to roll a certain way, I
don't think a simple expression of "good luck" conveys that meaning.
It's simply "may the dice roll in your favor." I can see, though,
where using it in this neutral sense might make it easier to slip
into using it the wrong way ("I think I'll wear my lucky shirt.").
Even in your sense above, where results of a school test should be
only a product of study, there is still an element of randomness in
things like which questions are chosen, events of the day that might
distract you, etc., so a friendly "good luck" still seems reasonable.
And anyone who thinks the stock market is deterministic...:)
How about "Heisenberg be with you"?