Teacher‑facing bilateral handgrip dataset in Year 5–6 pupils (9–11 years): writing‑hand preference, best‑of‑two grip, and Asymmetry % (Wales, UK)

De‑identified pupil‑level data used to support a teacher‑facing interpretation routine for bilateral handgrip in Year 5–6 pupils (9–11 years) in Wales (UK), collected across 18 primary schools. Variables include sex (boys = 1, girls = 2), age (years), height (cm), mass (kg), BMI (kg·m⁻²), right and left handgrip trials (two trials per hand, kg), best‑of‑two grip for each hand (kg), and writing‑hand preference (left/right/ambidextrous). Derived variables are included to support replication of the paper analyses in SPSS, including writing‑hand‑coded Asymmetry % (signed and absolute), stronger hand (left/right/tie), alignment (writing hand matches stronger hand), and decision‑band categories (≤10%, 10–20%, >20%).