Would not the last line be:
“The Class has lost the Class’ memory of grammar rules”?
GRAMMAR SLIDE
No, the rule is to add an apostrophe plus an “s” to the end of the word to form the possessive. There are style disagreements about the rule when dealing with proper nouns, e.g.. James, Louis, that end in “s”, but common nouns take possessive form in the way I described.
dog dog’s
family family’s
class class’s
And the apostrophe business is, apparently, what causes people to be confused when using the possessive form of “it”.
There is no apostrophe when forming the possessive of a pronoun.
Thus
I broke the table, so it’s missing its legs.
it’s = it is
its = the possessive form of “it”, referring to the table