I also thought of using a switch, but as you can see by the comments here, using switch(true) and then doing evaluations in each case is kind of an anti-pattern that can be hard to read and debug.
I would avoid and look for one of the other conditional options in this board.

To further clarify for those still confused on why passing in switch(value) doesn't work, it's because it's comparing a Boolean to a numerical one, which will always be false. Say our average is 95 and we're assigning that to a variable avg. If we try to pass in avg as the argument to the switch statement's parameter, this is essentially what is happening:

switch(avg) {              //avg == 95
   case (avg >= 90):         //comparing 95 to true, which is false
     return 'A';

(avg >= 90) is returning a Boolean. We'll never reach a switch statement that evaluates to true if we're passing in our avg variable as the argument. I hope this helps!

In your second example, you're dividing by 4 for the average instead of 3. The correct calculation would result in 5.33 which is greater still, than yourPoints.

Adding your points to the list can't change whether you are above average of not.

Because we are looking for one of the case condition results to be true :) Like, we have 87 as avg, so case (avg >= 90) gives you false.. next gives you true, so it will be used.