As a college student you must set priorities. Often, it is difficult to manage friends, employment, hobbies, family, illness, car troubles, weather conditions, learning, grades, scholarships, etc. Fifteen or more credits may be too many for some students depending on their individual classes and personal situation. Don't get over extended. Only you can decide what is most important to you at this time in your life.
Life is full of decisions, sacrifices, and opportunity costs.
It is your responsibility to do it correctly. Grading is also discussed on this page.