OK, folks, I can’t take it anymore.
When you are abbreviating a number or anything else and use the apostrophe, DON’T just hit the apostrophe once and leave it like that, to wit: Class of ‘13. When you do that, you look like an idiot. (I am referring here to newspapers, web writers, and the printer who produced the graduation announcement we got in the mail.)
Hit the apostrophe key TWICE, thusly: ‘’13; then DELETE the upside down one. That will leave you with the correct form: Class of ’13.