If you overload your students with too many lectures, they will be tired! (red battery icon above their heads). If students are not motivated enough, they can drop out.
You can reverse this situation if you upgrade the leisure spaces and their items: garden, gym, cafeteria... Pay attention to those items that could increase the students' energy and happiness/motivation (green smiley icon)!
If you tap on the stats button (upper-right corner icons) you can adjust the cut-off mark (minimum grade required) and the study programs according to your strategy. If you are too demanding, you probably could lose the flow of new students.
Make sure that your current levels in your university could handle your minimum grade and your academic difficulty; If your students start to drop out, or you can't find any more pupils on your campus, please readjust those bars. You can do it as many times as you want.