Sentence:  Imagine a country where children do nothing but play until they start compulsory schooling at age seven. Then, without exception, they attend comprehensives until the age of 16. Charging school fees is illegal, and so is sorting pupils into ability <WSD>groups</WSD> by streaming or setting. 
Meaning of the word marked with <WSD> tags:  A number of things or persons being in some relation to one another.