Grades have nothing to do with IQ. Well, what I mean is you can have a high IQ and get horrible grades. Grades have many components, doing the work, applying yourself, reading the materials, paying attention in class.
IQ and knowledge are related in the sense that people with higher IQ’s can attain and assimilate knowledge better than people with low IQ’s. Their ability for logic, how things are organized in their brain, their filing system in their brain is different, more complex.
People with average IQ’s can still be very successful, IQ is not a singular determinar for being successful, not even close. Average IQ paired with ambition, hard work and drive, probably does better than a lazy genius.
People with low IQ’s have it harder, depending on how low, it can be like not being able to understand certain things past the 8th grade level, or take much more time for something to make sense.