It does indeed say 去年 in the book. But depending on the exact usage of “until”, it could mean up to and including the time specified. I will admit that I don’t understand the exact nuance of まで temporally, and reading a Japanese definition didn’t help. But the example in Goo is 「明日まで待ってください」, and I would certainly expect this to mean “please wait until tomorrow” inclusive of tomorrow. So taking that back to the sentence from last week
こんなふうにカーテンを引いて、部屋で、身を硬くしている平日に見るものではなかった。去年、までは。
I think it’s reasonable to interpret this as 見るものではなかった was up to and including last year, implying this year is different. Also, I think it’s reasonable to assume all this talk about years is really about school years. It wasn’t something that happened last school year. This school year it is.
Here’s more from the very end of the first week’s reading that I think supports what I’m saying.
家が近い東条さんは、こころが学校に行かなくなってからも、毎日のように学校からのプリントや手紙を届けに来る。
とても、事務的に。
仲良くなれたらいいと思ったし、仲良くなれそうだったのに、プリントをポストに入れるだけの東条さんが、そこから一歩進んでこころの家のチャイムを鳴らすことはない。ただ義務を果たすようにプリントだけ入れて去っていく東条さんの姿を、こころは何度かこっそり、自分の部屋の窓から見送った。
青緑色のセーラー服の襟。臙脂色のスカーフ。四月には自分も着ていた制服。 それを、ぼんやりと眺める。
I think that last paragraph pretty strongly supports it being only a month since she stopped going to school. Since it’s already May, 四月には自分も着ていた制服 could only reasonably be referring to the April immediately prior to the current date. That is, last month. If it was the previous year’s April, it would have to say something like 去年の四月には自分も着ていた制服.