Here's a nice graphic and summary that includes GT in the tiered system of supports. The exact percentages shift somewhat in different sources. While RTI (Response to Intervention) is usually discussed in the context of struggling learners, it (statistically) applies equally to advanced learners:

http://www.cde.state.co.us/rti/LearnAboutRtI.htm

The important points to understand:

1. Tier 1, or core instruction, (AKA, the curriculum as presented by the textbook publisher) is expected to be sufficient for about 80% (70-90%) of the school population. This is as far as classroom teachers generally can differentiate.

2. Another 5-15% will be successful with some moderately different additional programming or supports. Perhaps extension assignments here and there, or a once a week pull-out enrichment period. A few of these kinds of support may be manageable by the teacher, inside a general classroom, using, say flexible or leveled groupings. Some textbook publishers do include supplementary materials that address the needs of some of these students, especially those closer to that middle 80%, and a few exceptional teachers are able to provide this level of support in their large classrooms.

3. 1-5% will need substantially more modification in their programming. It is unrealistic to expect typical classroom teachers to be equipped to provide these services.

Tamara Fisher's excellent blog on the topic (http://blogs.edweek.org/teachers/unwrapping_the_gifted/2009/08/rti_and_the_gifted.html) notes that a double ended RTI pyramid looks much like the bell curve, such that the top 2.5% need intensive instructional modification as much as the bottom 2.5% do--both are equally out of the range of tier 1 classroom instruction.

So just as RTI for struggling learners recognizes that classroom teachers cannot differentiate for the needs of the lowest 10 to 15% without assistance, it is unrealistic (and unfair) to expect teachers to be able to differentiate easily for the top 10% (let alone the top 1%). They have not been provided the tools to do so.

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