Most contemporary curricula are written for three levels of differentiation within the classroom, designed to capture roughly +/- 1 SD. Below that, RTI tier 2 programs (typically small group pull-out) should be capturing an additional 10-15%, and tier 3 and special education the remaining 5-10% (with overlap at each of these levels of intervention). Sadly, the other end of the double-ended RTI triangle rarely exists, especially prior to high school.

The plan Tigerle describes is essentially (plus some whole grade skips) how most members of my sib group went through much of elementary and secondary school, after extensive parental advocacy, and some media attention. On a note of optimism, one of the schools continues, decades later, to allow a fair amount of cross-grading (both directions) on the grade 6-12 campus.

...pronounced like the long vowel and first letter of the alphabet...