I would nicely remind them that they are not "making an exception" for your son, they are meeting his academic needs, which is an integral part of any school's goals for every child.
As far as the "line of other parents," remind them that very few children are presenting the way that your DS does, and that, while they may have parents who request evaluation, that does not mean that they will have a line out the door of children who qualify for advanced work. Furthermore, there is a widely-held misconception among uneducated people that acceleration is harmful socially, so there are unlikely to be many parents who would want to "do" that to their child. FWIW, when my DD was the first to subject accelerate within our district, there was an increase in requests for evaluation for a short time, but once enough children were evaluated and didn't meet the high standards of achievement, ability, and social concerns, the interest reverted back to occasional, at best. (And those who did meet the standards began to have their needs met.)
I'm glad to hear that you told the principal that their new plan was unacceptable. And I'm glad that they are regrouping and will get back to you, rather than just dismissing you outright.
Perhaps a next step for you would be to provide them with information about who exactly your psych is. Maybe also provide them with some articles addressing the non-issue of gaps.
