Use the apostrophe before an "s" to indicate the possessive (i.e. something that is of or belonging to something else – the graduating class of Memorial).
If something is descriptive rather than possessive, it does not take an apostrophe (the guide for students, the college for teachers, the point guard for the Sea-Hawks).
In cases where the noun ends in an "s", use a single apostrophe after the "s".
Note that the "s" followed by an apostrophe reads awkwardly, so it might be better to rewrite the sentence.
In cases where the possessive is plural, use a single apostrophe after the "s" or rewrite the sentence.
Use an apostrophe with plurals of lower case letters.
Capital letters and numbers have no apostrophe before plural-s.
Do not use an apostrophe with the possessive pronoun "its".
Use an apostrophe with the contraction meaning "it is".
(See the Possessives section.)