I wouldn't think so. They might get their first choice of specialty within the army as a young second lieutenant, and get some compounding advantages from that. But twenty years later, when you're up for promotion to general, aren't they going to look at your achievements as a colonel rather than wave you through on your long-ago test scores?

The pool of professional officers was very small and completely insufficient for the needs of the expanded military.

Being a semi-competent officer with some experience basically guaranteed that you’d be fast tracked to a senior command.

e.g. Grant left the military in 1854 and was just a captain when the war started. In about 4 months he was already a brigadier general

Promotion to general is very much a political effort as well as job performance today, but even more so back then.

Being at the top of your cohort from the beginning has a number of advantages. Your name is known from the very start. Big things are expected and big opportunities are presented. There is probably a very strong personal commitment to a military career by those bothering to achieve top scores, and not being a career captain, but aiming for colonel or above.

Being the top of the class also puts a person on a track that is even promotable to the very top. A 2nd lieutenant supply officer just isn't on the same trajectory as a high profile infantry officer commanding a forwardly deploy combat unit.

The class of 1915 had 36% of its 164 graduates go on to attain the rank of general. John Keliher (rank 159) made it to Brigadier General.

See: https://en.wikipedia.org/wiki/The_class_the_stars_fell_on