I think a C is okay if the other grades reflect any sort of difference.
The reason I say this:
I had problems with math all my life. From 3rd grade on I had C's and D's in math. No matter how much I was beaten or grounded I could not make any higher in math.
I can honestly say any time I am given any sort of a higher math problem to solve I freeze up and my brain turns to jello because I hate math so much. It made me feel stupid and worthless all my growing up years. I hate math.
I am near perfect in testing in basic math by the way. In any testing that is basic math like adding, subtraction, multiplication, division, fractions, all that...I test out with near perfect scores. So I am not dumb in math, it's the higher stuff I can't get, I have a glitch when it comes to that stuff.
On my ACT when I was a senior in high school I made 20's in every area except math. My score in math was a 7, yes a 7.
I did some evaluations in college as an adult and got a learning disability "diagnosis" based on my abilities to do math. I could have un-timed tests, open book tests, notes, a calculator, what ever I needed to succeed. They could not logically flunk me in any math class I was required to take.
So my point is, they may have some sort of glitch in this area and it's not diagnosed. It could be that the light bulb just has not come on yet too. Some areas of the brain take longer to wire up and turn on. One day that light bulb may come on and that particular subject is easy and the grades go up.
I think if there is a drastic difference in their grades then it might be a good idea to talk to the teachers and find out if they seem to be struggling with the concepts or just hate the subject. Perhaps taking them to an educational evaluation to see if they can find some glitches or possible missing links would be in order. They may need to go to some special tutoring for that particular issue too.
There are many options before just saying they are not doing their best. They may absolutely be doing their best and learning they are stupid and worthless in the mean time.