I wish I could say I had not heard this line of reasoning before, but it was exactly the reason I was told by my supervising teacher to not allow the gifted boys in my fifth grade class to experiment with the materials for the science unit we were learning at the time. "If they learn everything in fifth grade, what is their next teacher going to do?" It's very similar to the "if they test out of the entire spelling list by January, then they'll be bored until the end of the year." (Like they wouldn't be bored if you hold them back?)
Both are stupid arguments. Educational "best practices" say schools should be child-centered, not teacher-centered. *No child will learn at the teacher's convenience every day.* Some will learn "too quickly" all the time, some will learn "too slowly" all the time, most will learn either too quickly or too slowly depending on the day, the subject and the way the subject matter is presented. Most will figure it out if she goes over and over the material in different ways over the course of many days. This is the basic premise behind classroom teaching.
I'll save my rant about how completely arbitrary school procedures are for another day, except to say this: There is no magic age at which a child "should know" a particular concept in any subject, no matter what E. D. Hirsch's books say. State educational standards are usually written to require mastery of a subject several years/grades after the subject is first introduced. This is why we have consolidation years in which little or no new information is taught. (The consolidation grades are generally 2nd, 4th, 5th, 7th, 8th, 10th and 12th. Yes, this is half the usual 14 year school career--including pre-K and K.)
This is one reason why gifted kids present a "problem" for schools. Many have no idea what to do with kids who hit state mastery standards the first time around and want to keep learning. The schools are set up for that. They're set up for "normal" kids who need 15-18 repetitions for mastery of a concept. Gifted kids who get the concept after 1-3 repetitions spend a lot of classroom time being bored, even if you don't "teach ahead," which does make the boredom problem worse. (In case anyone is wondering, "bright" but not gifted children usually need between 6-10 repetitions for mastery.)
This is why we left the public schools. Kids only learn when they're ready and willing to learn, not when some textbook publisher or state standard says they're supposed to.