校 - How did "leg-brace" come to mean "school"?
Outlier

xiào
○ school

Here the ○ icon is an indicator of a phonetic loan. So, basically it is just a:

character that is &quot;borrowed&quot; to write another homophonous or near-homophonous morpheme

It seems that the earliest reading was jiào and not xiào; 校 was just borrowed for its close proximity in sound.