First, the parent should consider the level of problems the child is having and whether they will be easily overcome or remedied in the next year. Supports can be put in place to assist the child at home or within the school system.
Secondly, Social concerns are often something parents worry about. The stigma form the parents peers or from the childs peers is often a roadblock in the parents mind to keep them what they think might be better developmentally for the child. In many cases parents should remember that the social disapproval from other peers will probably be forgotten very quickly and may not be worth years of the child having problems academically and socially. If the child has another year to develop he may be far in advance of his classmates and academics and socialization will be a joy for him and very easy.
Thirdly, if the child is placed in a special classroom and need additional help this is often helpful if the cild has severe problems. However, many cases in which a child that has only few issues that could be resolved with intensive ABA and delaying the move to the next grade. The child may have a very small chance of being promoted out of a special education class. The actual statistics (or the approximate number per year) of your school for how many children are put back in the normal classroom can be attained from the school.
In most cases with very strong supports from behavior therapist (ex. CBA or BCBA) children with mild autism can often be mainstreamed in the normal classroom. In general, the level of supports, level of functioning of the child should be considered.
The present discussion provides some general points for you to weigh in making this difficult decision of whether to move your child up to the next classroom and not a formula for every child. If the numbers are very low for mainstreaming in your school then the stigma of being in a special classroom may not be worth it. It may be better to have your child be the tallest smartest kid in his class by delaying his enrollment in the next grade. People will forget about the problems once they see how wonderful your child is progressing and moving toward attaining his degree in the regular classroom!