While grades in school can certainly influence your ability to succeed in your career of choice, they are not a 'great' influence, and the influence they do have is very indirect.

While I'm not discounting the possibility, I have not once heard of a prospective employer wanting to review a students high school transcript before making the decision. Some prospective employers want to review ones university transcript, but this is as well is far from the norm.

So really, school grades primarily influence ones ability to attend the college of ones choice- and even this is indirect. Good grades can help one attain scholarships, and can help in the application process to colleges and universities with some measure of exclusivity; as such, better grades can make it easier and cheaper to get into the same university, and can make better universities more accessible.

A student with worse grades will likely have to pay more for school, may have to wait a semester or two longer to get in, may have to attend a less desired school, etc. But the end result, assuming you successfully complete your degree, is largely the same.

Ultimately, poor grades in school may cost you specific individual jobs very early in your career, but aren't likely to change it's path significantly. For the most part, solid experience in ones field and a history of good performance weigh far more heavily than what school your degree came from.