Grade x vs xth grade

Solution 1:

There may very well be dialect differences at play.

I've found that in the US, both terms are used regularly, though I think "xth grade" is the more frequent. Especially in informal speech, such as what you might expect when asking a child "what grade are you in?" The answer will almost invariably be a variation on "5th grade!" The other, I think, might be more likely found in a more formal (likely written) register: "The new Snooze and Blattmann series of Vosnian language text books, suitable for grades 10 through 12, should be available before the school year starts."