What a metric "School Speed Limit Ahead" sign in the United States would look like. I have been unable to find any diagram for such a design in either the current or older versions of the MUTCD, therefore it can be assumed that such a sign is unauthorized. The situation is complicated however by the fact that the MUTCD requires "speed limit ahead" signs to reflect the same speed that is ahead, so if a metric school speed limit sign were used, this design would be required by the MUTCD's own guidelines.
This file was derived from:

 MUTCD S4-5.svg: 
MUTCD S5-1 METRIC.svg:
MUTCD_S4-5.svg:
MUTCD_S5-1_METRIC.svg: Federal Highway Administration - MUTCD
derivative work: Fry1989