In a case like this, most likely Wolfram|Alpha just doesn't have the data you are looking for.

If you want to have more control over your queries, you have to use Mathematica. The query above translates into the following code:

EntityValue[Entity["Country", "UnitedStates"], EntityProperty["Country", "PopulationBySchoolEnrollment", {"Enrolled" -> "9To12"}]]

If you look at the Entity Properties available to Entity["Country", "UnitedStates"], you'll see there aren't any for individual years.

If you would like to see this data in Wolfram|Alpha, you can leave a comment on the website wolframalpha.com. There is a team of curators who will be glad to file your suggestions.