Since 々 is not a kanji in the first place, there is no rule regarding in which school year it is taught. The official kyoiku kanji list does not mention this symbol at all.
Still, there are many easier two-kanji compounds including 々, such as 人々 and 時々, and 四字熟語 is a relatively difficult set of words no matter how easy their kanji are. For example, you cannot assume a first grader knows the meaning of 三々五々 even though 三 and 五 are very easy as kanji. It's hard to imagine someone who can read a 四字熟語 does not understand 々. If you are specifically working on 四字熟語, I think you really don't have to worry about the "grade" of this symbol.