In class years and decades, the apostrophe should point to the left:
’02, P’00, GP’89
’80s, ’20s

There is no apostrophe in a range of dates: 1985–89

Note: According to Chicago Manual of Style Online Q&A: “In word-processed documents, when apostrophes are preceded by a space (as opposed to those in the middle of a word, like it’s), the software thinks the writer wants an opening quotation mark and supplies one. When documents aren’t proofread carefully, these marks appear in place of apostrophes.”

Avoid using “daggers”: A dagger is a straight, pointed character that can be used as a reference mark:

Not OK:  '80
OK: ’80

How to make a left-facing apostrophe: This character is located in Microsoft Word’s “insert” menu > symbols > normal text > special characters. Select the “single closing quote.” PC users, creating a shortcut is helpful if you use the character often. Mac shortcut:  shift + option + right bracket key