Grades. We've been receiving letters to define how well we're doing in a class or course since elementary school. The stress to have an A or a B tells us mentally that we're smart, or intelligent. Can someone be successful with C's? Yes! There's more to you than grades. Just because you're not good at school doesn't mean you're not good at anything. School is a standard that doesn't apply to everyone, and that's OKAY. It's like asking a turtle or an elephant to climb a tree---not every animal can climb a tree, but they contain other skills that are valuable to their overall existence.
A number of presidents did poorly in school at one point or another, including John F. Kennedy, Lyndon B. Johnson and George H.W. Bush. Joe Biden also struggled with his grades as both an undergraduate and a law student.
In addition to some of our country's leaders, there are a number of incredibly successful entrepreneurs who didn't allow their academic experiences to keep them from coming out on top. Steve Jobs, for example, never finished college. Bill Gates and Mark Zuckerberg also didn't complete their college years.
Elizabeth Holmes---who is revolutionizing medicine---dropped out of Stanford to pursue her dreams. Simon Cowell wasn't always the top dog as well. Cowell left school with just one O level ---the equivalent of GCSEs--- and dropped out at just 16.
Academic achievement is not always a proper way to measure how subjective your intelligence is. Success as a student is largely dependent on one's ability to operate within a certain system, but it's not always the best preparation for the real world.
A person's character, emotional intelligence, perseverance, experiences/connections, and the ability to understand failure are all key to success.
This doesn't mean receiving poor grades means you'll automatically become successful, but getting good grades also doesn't mean you'll end up on top. Don't be discouraged if you're doing all that you possibly can and still not doing well in school. There's a path for you - all you have to do is get a good education after you leave the classroom.