Final answer:
The correct punctuation to add is commas before and after the phrase 'who were all blonde.'
Step-by-step explanation:
The correct punctuation marks to add to the sentence are commas. Commas should be placed before and after the phrase 'who were all blonde' to set it off as a non-essential clause. The sentence with the correct punctuation would be: 'The high school cheerleaders, who were all blonde, were the most popular kids in school.' This punctuation clarifies that all high school cheerleaders were blonde and indicates that this information is not necessary for understanding the main point of the sentence.
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