...I actually have one more troublesome sentence, if it's not too much of a bother.
What messes me up is the recurrence of 明文化, which translates into "stipulate"...there are no other options. I tried replacing it with "decision" at first, but decided it didn't fulfill the role like I thought it would. When I try to put it into words, I just can't get it to work.
Also, the third clause doesn't seem to connect with the second at all. Maybe I mistranslated something?
今までも指導要領に詩歌の創作は明文化されたことはあったが、小学校だけでなく中学校にまでも明文化されたのは、昭和二0年代の試案のものを除けば今回がじつははじめてである。
Even until now there was a decision for poetry composition in governmental guidelines, which specifies that it's not only elementary school but up until middle school, if you sneak a peek at the 1945 tentative plan, this time is actually the first.