It's not enough to just let students take the classes at their level; you also have to offer them classes at their pace. The 5th grader who's ready for algebra is probably capable of finishing the course in a third of the time that it would take a 9th grader who is taking algebra at the usual age for US students (at least, that was the case for me and the cohort of segregated gifted students I was in).
Offering special classes for gifted students can also help solve a lot of problems stemming from disparities in emotional and academic maturity, or offer instructors the chance to provide greater depth or branch out beyond the standard curriculum.
Offering special classes for gifted students can also help solve a lot of problems stemming from disparities in emotional and academic maturity, or offer instructors the chance to provide greater depth or branch out beyond the standard curriculum.