I'm too lazy to go the Google search, so I'm just going to go off the dome and may be incorrect.

A lot of the research regarding breakfast being important, and justifying free breakfast programs in schools, stemmed from measuring calorie deficit in overall terms instead of satiation. Namely, if kids were hungry in the morning, you should probably feed them since they likely are running low on energy, but a kid saying no to breakfast wasn't really studied.

I think there were also some studies which indicated that a carbo load similar to endurance athletes may show an improvement in test results. This has led to some teachers and schools getting a bagel day organized during test day so students have an opportunity to carbo load before taking a test. Where carbo loading doesn't lead to increased test taking abilities hasn't been studied, so it is unknown whether the improvement is in filling an existing bucket or overfilling the bucket beyond what it normally would take.

Other research may be out there to confirm or deny my recollection, but it seems like complex carbohydrates may be the best option available.