Where does “小一” come from?

小一 (Japanese) comes from Japanese 一, from Japanese 始め, from Japanese 始める — begin; start; initiate; originate.

小一 (Japanese): first-year student of an elementary school

Definitions

小一 traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Japanese	一	one, 1; "term relating to the number one"; unity
2	Japanese	始め	beginning, the start of an activity or time...
3	Japanese	始める	begin; start; initiate; originate

Step	Language	Word	Meaning
1	Japanese	小	smallness; small size, small; Short for 小の月;...
2	Old Japanese	ささ	—