Here's some info for you:
First is a memorandum of agreement between DOE and UFT
Here we have the new construction classroom space guidelines as per NYSED.
Current NYC building codes. Scroll to page 166 to see school room square footages. Note pre-k and K require 35 sqft.
And this. An interesting read showing how low even 35 sqft for pre-k and K is when compared to many other states.
I would imagine if actual research were done very few K classrooms meet the 35 sqft criteria. The ones in my school sure don't.