Dear aspirant, 

If you have got low percentage in class 12th , it will not affect your placements unless if you do well in your bachlor's degree. If you got good marks in your bachlor's degree you will definetely get a good placement in a company . The company checks your resume , not your marks. However they will only check your bachlor's degree marks for eligibility purposes and nothing else. If you go for an interview you show your resume and skills , not your marks . Don't waste your time and life by just repeating 12th std again and again . Learn to  do things right ahead , learn from your mistakes , check where you are lagging  and make a note of it and learn from it  . So better make things right ahead , forget the past , it is gone , it will never come back , we can just learn from it and make right ahead .