I just finished reading Volume 12. I heard that the series is ending with this volume, but couldn't find any confirmation in the volume, since there's no author's note at the end. Do you know if this is really the last volume(and if so, if there's a source), or whether it will keep on going?

The author confirmed separately that Volume 12 is not the end, though he previously had planned it to be. Volume 13 is coming out at the end of this month in Japan I believe, and the author stated he's planning to likely cover up until their high school graduation, or possibly just turn the series into a lifelong work.

So in short, it's definitely not ending lol