If your true passion is teaching, then I'd say mastering the material you're given to learn is ESSENTIAL.
I can also tell you with the No Child Left Behind federal program, public schools now have new standards for teachers (if you planned to teach in the US). A C or D may not be good enough for the guidelines, even if you can get an employer to look beyond a low GPA.
Instead of looking for reasons to rationalize a poor GPA, can we focus on ways to help your GPA?
For example, what do you mean by "competition"?
Why do you feel you don't have time to study? Are you having personal problems or a work schedule conflict? Can you drop one class in an effort to get an A or B in the other classes?
What are you having the biggest problem with? Specific subjects? Time management? Lack of interest in learning in general? Or ???
I do feel your pain. I'm back in college for a new career myself. It's like trying to memorize sections of a dictionary sometimes.... blah.

but it can be done. Don't do it half-assed and have bad grades haunt you for years to come.