Time dependent! Let's break this AWESOME question down into a few AMAZING parts that are most INTERESTING!
How long do you have to study? Months, weeks, days, hours?
Months (6+): TDCS, choline, piracetam, combined with spaced repetition! Add in the following before every meal: thiamine, folic acid, L-glutamine, potassium, sodium, magnesium, and a bit of ye olde glucose for fun! Have you been drinking? Might as well fill up on the B vitamins (and a rhodiola or some centrophenoxine for good measure!) Be careful and pace yourself... you have time to get those numbers right! Don't take too many pills at once!
Weeks (<4): Shit son, you better keep up that spaced repetition (especially with the spanish test!) Increase your potassium intake if you can (and add the rest mentioned above, but watch your digestion!) Let's start adding in some caffeine just to make sure you'll be ready for the big game and won't have a headache or the jitters.
Days (<3): Too late to experiment with IDRA-21, the PRL-8-53, or even the gorgeous TDCS! You're in this and you need to stabilize! Get your salts right, your diet bland, and your water intake nominal. Read, and get down to base level. You're gonna want to pop that nitro when you need it, not before!
The Day (T-minus, 2 hours): Breakfast (if you do that thing) make sure to leave out the fat. Fat makes you sleepy. Keep away from lunch if you can. Glucose (sugar, bread, dextrose, etc.) is good for now. Maybe some peanut butter, protein powder, etc. Right before you walk in, take your modafinil. That will probably make the most difference. Adrafinil is OK too, if you check the dosage. As a last resort, do some DMAA, but don't combine that with anything else. Long story short: sugar, caffeine, and nicotine are going to do well if you get a good supplement (modafinil, adrafinil, DMAA, Dex, etc.) because they tend to amplify whatcha got already in your system.
Want to do well on the day of the test? Sleep in, smoke weed, get a coupon for pizza and order takeout. Nothing's gonna save ya. But at least I'm honest.