Incorrect spelling, explanation: possessive form for regular plural nouns is only the apostrophe at the end: here boys’ is correct. This is because these forms already end with s, so we do not add another. Therefore boys’s is the wrong form.

Correct spelling, explanation: ’s is the way of writing down possessive in English, however only in singular form (Kate’s, techer’s) and irregular plural forms (children’s). In regular plural forms, thus in forms ending with the consonant s, we do not add another s, just the apostrophe. Therefore the correct possessive for boys is boys’, not boys’s.

Definition of boys:
noun, possessive, belonging to two or more boys
Sally prefers to wear boys’ hoodies. She says they are warmer.
Are those your shoes? – No, they’re boys’.