In fifth grade, my teacher would always rush through our math lessons and give us a test almost every week. I would sometimes get lost in the middle of a lesson and not understand at all. The next week we received our previous tests, and I had flunked it! It had lowered my grade from an A to a C. Therefore a student who fails a test should be aloud to retake it for maybe half the points.
If a student can not read or write or do basic math, then a 60 percent might be reason for repeating,although not without some attempt at coaching because it holds a young person back socially. Other subjects that are not as crucial should have a much lower threshhold before repetition is necessary
If you get below a 60% you should not have to repeat the grade. Repeating the grade serves as an embarrassment, lowering of self esteem, and a loss of friends. Instead of making a child go through that there should be other options. Perhaps requirements to take extra classes to learn the stuff would work.