The natural increase of homework and difficulty in classes throughout the year, peaking near the end of the third quarter. This is due to the increase in difficulty of the curriculum and the compilation and compounding of concepts. The peak comes before review for finals, when the last few sections of material for class are given to students.
Also, many projects and term papers are given or due near the third quarter peak.
My grades gradually dropped as a result of the third quarter effect this year.
I can't get into college because the third quarter effect ruined my GPA!