Ichigo should be in 10th Grade at the start of the manga.
In Japan, a student in 10th Grade turns from 15 to 16 so it was planned from the start assuming Kubo had Ichigo's birthday planned out from the start. Looks like there was just a minor miscalculation on Kubo's part.
Re: Has Kubo made another timeskip without telling anyone?
Originally Posted by Zeta42
When the story just began, Masaki had been dead for 6 years. After Aizen's defeat, 17 months have passed. We don't exactly know how much time passed between the beginning of Bleach and Aizen's downfall, but if we add that to those 17 months, we still can't get more than 2 years. Therefore, when the Fullbringer arc started, 8 years have passed since Masaki's death at most.
But now we have Chapter 537, which tells us Masaki died 9 years ago. So... What does that mean? Was there another large time gap between the Fullbringer and Vandenreich arcs? Or did Kubo mess up on this one?
I don't recall Kubo saying how much passed between Fullbring arc and the current one... Could be a year for all we know