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if a child has an IQ in the “high average” (but less than 115), does the Iowa Acceleration Scale say, suggest, or mandate that the child should not be considered for Single Subject Acceleration.
No, I do not find that type of recommendation in the IAS.

The IAS is subtitled, "A Guide for Whole-Grade Acceleration K-8", therefore not addressed specifically to single-subject acceleration, (although single-subject acceleration (SSA) is mentioned briefly in the Manual on page 104, and Planning Record page 3). Some schools may use the IAS to prepare a complete record of a student's scores and "thoroughly discuss the case and to consider other curricular options and recommendations for the student." (Manual page 34)

While an IQ of at least one standard deviation above the norm is considered one of several critical factors for whole-grade acceleration (Manual page 35; Form page 4), it is also stated that a student performing above the 50th percentile on above-level material may be ready for "more challenging material" (Manual page 120). Consideration is given to the student being successful under the expectations of the new environment and also being successful in the long term (Manual page 5).

Page numbers were provided above so that interested parties may read the selected statements in context.

To prepare for advocacy for single-subject acceleration, you may also wish to look at the book and forms by Dr. Karen B. Rogers.