When I was in school, teachers constantly warned us that if we didn't get good grades, we wouldn't be able to get good jobs. Parents (including my own) often said the same thing.

Getting good grades in school wasn't only seen as a prerequisite for professional success. Good grades were -- and still are -- assumed to be a predictor of long-term career success

Or not.

Tom Corley, an accountant and financial planner and the founder of Rich Habits, surveyed a number of high-net-worth individuals, many of whom are self-made millionaires.

Did most of the self-made millionaires get great grades in school?

Nope. Only 21 percent of the self-made millionaires were A students.

Forty-one percent were B students. Twenty-nine percent were C students.

In short, more of the self-made millionaires were C students than A students.

I know what you're thinking. Family backgrounds played a key role. Money comes from money, right?

Not really: Fifty-nine percent of the self-made millionaires were brought up in middle-class households. Forty-one percent were brought up in poor households. None of the people Tom surveyed came from upper-class backgrounds. (Granted, that's not always the case: Of the extremely wealthy people I know who are self-made, at least a few came from affluent families.) 

All of which proves that where you start does not have to dictate where you finish.

Success in life does not come easy. It is fraught with pitfalls, obstacles, failure, and mistakes. Success requires persistence, mental toughness, and emotional toughness in overcoming these pitfalls. Its pursuit pushes you to the edge emotionally and physically. You must grow a thick skin and become accustomed to struggle if you hope to succeed.

Individuals who struggle academically may be more accustomed to dealing with struggle and making it a daily habit to overcome pitfalls.

In short, people who struggle early on get the opportunity -- and it is an opportunity -- to become mentally tough and build a foundation for long-term success.

What matters more than grades?

Most successful people are great at delaying gratification. Most successful people are great at withstanding temptation. Most successful people are great at overcoming fear in order to do what they need to do. (Of course, that doesn't mean they aren't scared -- that does mean they're brave. Big difference.)

Most successful people don't just prioritize -- they consistently do what they feel is important.

All of those traits require discipline, focus, and determination.

So if your goal is to someday be wealthy and you didn't get great grades in school, that's OK. The past -- especially your school past -- doesn't define you.

Plus, "school" is really never over. Successful people are lifelong learners.

It is now clear that one's IQ can change over their lifetime. It's not fixed. Just because you were a C student at age 17 with an IQ of 100 doesn't necessarily mean you will stay that way. You can increase your IQ all during your life, even into your 80s.

Self-made millionaires do certain things every day that improve their brains and continuously increase their intelligence during their lifetimes.

And you can too.