Think BIG:
Sure, some subjects are inherently difficult, and may cause frustration to students. But what message does it send to not teach them as a result? That if something is difficult and frustrating, it should be abandoned?
The better view is probably that the students should be pushed harder, and taught better, until they are at a level where they excel and are no longer frustrated. I’m sure reading is often frustrating for young children, but we don’t stop teaching that.
Even if much of the material is re-taught in college, having to deal with the material in high school not only teaches work ethic and forces better study habits and learning skills, but it also sinks in more than you think. I cannot tell you how many times in college I took a class that covered material substantially similar to that which I encountered in high school, and found that I remembered more than I thought. As much as you don’t realize, you absorb knowledge and information even though you think you’ve forgotten it all. It comes back out when you are again confronted with similar material.
Gavra:
I would agree that a “B” is a good grade – it is above average. However, I believe that there is no such thing as setting goals too high. Not to the point where a kid gets in trouble if he/she does not get an A or A+, but every child should feel that they can do better, and push themselves. That is the way you improve – not by accepting what you already have. I thank my wonderful parents constatly for always pushing me and telling me I can do even better than I was (and I was doing pretty well). I hated it at the time, but in the end, they were right and I would not have learned it if not for them. It should be mentioned that all of this is relevant equally, if not moreso, to limudei kodesh.