"Boys'" is the plural possessive form, so it would be used like this:
She packed the boys' lunches and set them on the counter.
The big boys were ruthless (mean) to the little boy.
Maladjusted young boys can often turn to crime.
The boys obnoxious attitude did not jibe with the other students.
A sentence using the word faithful is: The boys have been faithful fans of the team since they were toddlers.
Example sentence - The boys traded baseball cards as a hobby.
The boys' locker room was so fetid that no one wanted to go in it.
Whenever Pete was in the boys changing rooms, you knew he was up to mischief.
Yes, "Boys" is capitalized when it is the first word in a sentence or as part of a proper noun.
After 24 hours of playing basketball, the boys finally had to succumb to exhaustion and go to sleep.
The president is being impeached.
You could use the word aboniminable in a sentence like this: The boys' behavior was abominable on the school trip, and Alfred's behavior was abominable because he threw a chair at Hector. EDIT: The boys' behavior was abominable on the school field trip because Alfred threw a chair at Hector.
the boys table manners were dispicable