Most school zones in the US are 20mph (32km/h). Some states will have 25mph and, for example, a highway going through a school zone that usually 65mph may go to 35-50mph when the school zone is active but this is an exception, not the rule and is done on a case by case basis.

Remember though, American roads are much larger tha European roads. Most of these school zones that go that slow there's really no need for it.