Do I really need to take summer school if I got a C in one class?

No, not for just one class, unless you’d like to major in that subject or unless you are going for a really high GPA. Or, unless you feel it will keep you from doing well next year, then summer school can totally help. But re-doing a class is a no-fun way to spend your summer, so the best plan is to try to get the grade you want the first time around.