Time for some detective work!
Hints we have! The chapter is titled 5月. I think we can safely assume it is May at this time in the story.
Then there’s some further hints: 東条 starts school a bit late. It seemed to be described as a very short time, so possibly only a few days. Enough to be noticable, but not really enough to factor hugely into the time span.
The first 2 weeks after 東条 joined the class were fine, and こころ got along well with 萌. (P. 22 both)
Furthermore, when she meets the teacher at 心の教室 there’s this part: 入学したばかり。それなのに、最初の4月しか、あそこに行っていないのだから。(p. 16)
So we know she only attended school for a month her first year.
Finally, (or really firstly), on page 11 (right at the start!), こころ is musing about all the things she normally didn’t experience on a weekday. 去年、までは。
So we know it has been pretty much exactly one year since she stopped attending school.
Page numbers from the tankobon
Sorry if this already got answered. I had fun in any case.