Sure, but if merely switching compilers produced a faster binary, then I would expect all programs to be compiled with the better-optimizing compiler. After all, it doesn't take any particular expertise to adjust the value of CC.
There's more to optimization than setting -O3. Learning how various compilers behave and how their optimization features interact with your code are valuable skills and may well have been within the scope of the course. Certainly worthy of extra credit.
The class clearly has a performance component, and so students were expected to learn about optimization. Are they going to learn optimization better or worse if you mandate a single compiler? If merely switching compilers is the best path to performance, is that not a valuable lesson? If switching compilers and doing a bunch of extra work to make the code fast with the new compiler is the best path to performance, have they not learned a great deal?
Some compilers aren't generally available. Hypothetically, what if ICC wasn't available freely to educational users, but some of the students had side-jobs where they used it?
You can always mandate a large set of compilers, make them all available, and leave it up to the students to determine which is fastest. I think that acheives both the competitive/educational goal and the level playing field goal.
I would definitely ban using any compiler that wasn't generally available to the class, or at least disqualify their output from winning the contest. I'd take a generic approach where it's worded just like that, rather than trying to come up with an official set of acceptable compilers, though.
