@Ghang I believe that 偏差値70 would mean grades that are in the top 2% of students and the joke is she takes it to mean just scoring 70%s on her tests.
If you know stats, you can see the jdict definition for 偏差値 mentions a mean of 50 and standard deviation of 10, so 70 means two deviations above that, and the area under 70+ on such a bell curve is only ~2.3% of the total.