According to Timothy J. Lawson in his book, "Scientific Perspectives on Pseudoscience and the Paranormal," holding a child back only creates the illusion of success, when it actually causes more harm than good. If a teacher feels a student should repeat the grade and a parent agrees, then the student will continue the next year re-learning old information. Thus, it is evident that the student will have an easier time progressing through the year, because no new information is being introduced. So, to the teacher and the parents it would appear holding the child back was a success. However, when the next year comes around, the student will find themselves right back to the same level of difficulty in understanding, as if the last two years had not even happened. Holding back a child can cause harm socially because they will be away from all of their friends and may feel embarrassment in the later years.
Overall if a parent feels their child is struggling, the best bet is to enroll in a tutoring system and really helping the child along the way rather than, allowing him/her to repeat old information without gaining anything new.
Lawson, Timothy. J. (2007) Scientific Perspectives on Pseudoscience and the Paranormal. Upper Saddle River, New Jersey. Pearson Education.