Is it decreasing more than usual? As far as I know there's always a steady drop in the bar when not studying, in class, using the study objects, or raising the related skills.
It's a lot easier to have an active social life with only one course per term rather than three courses. Also the term grade seems to be based on the final exam, so my party sims have been able to skate through the first week of a double term, completely fail all three mid terms, then buckle down to earn an A in the second week.