校

Left tree radical - indexing radical for 校. Originally 校 meant a wooden enclosure or cangue (the X-shaped wooden frame used to discipline prisoners), then a wooden military camp, then a wooden teaching hall - wood underlies every sense. Same family as 桌 (table), 椅 (chair), 板 (plank). The school-meaning is the modern descendant of 'wooden teaching hall'.

Right phonetic 交 - supplies the sound (jiāo → xiào, palatal shift; same family as 较, 狡, 咬). Originally pictured a person with crossed legs. The crossing image sneaks back as semantic flavor: 校 also reads jiào meaning 'to check, collate, proofread' - comparing two texts side by side, where they cross.