Develop a new measure of overall academic achievement to take this into account. You should be able to implement your model objectively, without making any assumptions about which classes are "easy". That is, if you know the grades of each student in each course, you should be able to determine from that data which courses are more difficult.
Keep in mind that courses with higher average grades may not necessarily be easier; it could be that the students who take these courses are stronger.
Consider a city environment with roads, stop signs, and traffic lights. (and if you like add congestion).
Let top speed be the maximum speed it can travel (ie the highest speed a driver chooses to drive)
Let average speed be the total distance driven divided by total duration of a trip.
Make a reasonable model and mathematical simulation or derivation that shows how the average distance traveled per hour (average speed) varies as a function of the top speed. Plot a curve for different top speeds and discuss relevant cases (e.g how much faster is a car than a bike (assume a bike can go top 20km/h)? How much does speeding gain?
Validate your model on reasonable data (Real data, or data you generate that you can convincinly argue give realistic results.)