First year of elementary school (abbreviated as 'shō ichi'); refers to a first-grade elementary school student or the grade level.
来年から息子は小１になるので、入学準備を始めた。
My son will be in first grade next year, so I started preparing for his enrollment.
小１の算数は数えることや簡単な足し算が中心だ。
First-grade math focuses on counting and simple addition.
小１の頃の友達と今でも年に一度会っている。
I still meet friends from first grade once a year.