Game credits were developed to offer students a 90 second brain break. They were not meant to be collected and stored over time so that students can take LONG breaks from practicing and play games.
As a result, anytime a student logs out of GMM (closes their browser) they will lose all but 2 of their credits. In other words, students can only carry over at most 2 game credits from one practice session to the next. This should encourage your students to use them as intended, as a small break, and not a day off!