Normally the child must be six before the child can START first grade. Boys develop more slowly than girls and need the extra time.
First, I don't quite know what you mean by "Accepted". I'm going to assume that means that s/he was promoted to First Grade.
The answer is yes. The decision as to which grade the child should be in is simply ministerial. This means that the school can place students where they think they should be. Even back in the fifties, I remember when one of my classmates spent a week or two in sixth grade and was put back in fifth because the school did not feel that he was ready. There is no law governing this and the courts will VERY rarely become involved.
Realize that this decision was probably not made lightly because the school almost certainly knew that you would consult legal counsel.
