Disable automatic "ligature" handling in PDF/Preview on El Capitan
Are you sure it isn't a simple font substitution error? What font is the PDF rendered in? Do you have that font installed? Does the lab machine have it installed? Does the document you are pasting into use the same font as the source PDF?

I did a simple test using your sample text, placed in a Word document (with ligatures enabled), then saved as a PDF using either Apple's in-built PDF or Acrobat Pro.

The original text, cut and pasted from Word:
“This is different from those who can afford to pay for college first; such students may find it less difficult by at least fifty percent.”

Apple PDF, cut and pasted from Preview:
“This is different from those who can afford to pay for college first; such students may find it less difficult by at least fi,y percent.”

Adobe PDF, cut and pasted from Preview:
“This is different from those who can afford to pay for college first; such students may find it less difficult by at least fi�y percent.”

Adobe PDF, cut and pasted from Acrobat Pro:
“This is different from those who can afford to pay for college first; such students may find it less
difficult by at least fiy percent.”

All of the above attempts pasted correctly, with the exception of the "fty" ligature, which differed in rendering based upon the PDF creation and rendering method(s) used. This character likely didn't translate correctly into plain text because I have enabled more than just basic ligatures in Word (optional and discretionary, but not historical).

All in all, it looks to me to be nothing but a font encoding or substitution error. Remember that a PDF document will have any necessary font characters embedded in the file itself, but when pasting, the target font will not necessarily match the source, especially if you are using a system with a clean install (meaning fewer fonts).