Gifted Issues Discussion homepage Forums Learning Environments Research Against Ability Grouping?

I've never read anything against it that wasn't strictly political in nature.

Here's an interesting article I came across last week.
http://austega.com/gifted/articles/Rogers_researchsynthesis.htm

It's by Karen Rogers, the author of "Re-Forming Gifted Education." She looks at the "effectiveness" of a whole variety of GT-Ed provisions. Cluster & pull-out grouping come out near the top.

Sounds like your principal needs to spend some quality time with current articles on the subject.