In level 2 apostrophes we looked at putting an apostrophe after the s when something belongs to more than one person/thing, or a person or thing’s name ends in s.
Recap:
Tess’ coat. (The coat belongs to Tess and because her name ends in s we put an apostrophe after the s.)
All of the students’ books. (The books belong to more than one student so we put the apostrophe after the s.)