An academic credit was a point earned upon the completion of Starfleet Academy courses or other related activities. Accumulation of a certain number of these points were necessary to advance to the next grade level or graduate.
In 2365 and 2366, prior to being accepted at Starfleet Academy, Wesley Crusher earned points towards his future education by serving as an acting ensign on the USS Enterprise-D, although he felt this method to be different from studying. (TNG: "Samaritan Snare", "Evolution")
As punishment for the rest of Nova Squadron's 2368 actions in covering up their failed attempt of the Kolvoord Starburst (Nicholas Locarno having taken most of the blame and an expulsion from the Academy on their behalf), Admiral Brand gave Sito Jaxa, Jean Hajar, and Crusher a formal reprimand and cancelled their academic credits for the past year, causing them to remain behind a year in their studies. (TNG: "The First Duty")