If you have the classmates name and approx age I usually begin here:

http://www.veromi.net/processor.asp
(the top city is the last known location)
(you can also get relatives names and age)

If you have a very common name try to see if the county the school is in has a court record search
(here you may get middle initials and actual birth date)(some have marriage records!) I know Florida has a county search for names- ie: https://www.myfloridacounty.com/serv/MyFloridaCounty/ORI/Order?thisPage=MyFloridaCounty.ORI.Order.state.Start

Then I utilize:
http://pipl.com/
(pay close attention to phone #'s that come up under googgle link or white pages) Zabba seach is not the best but does come in handy if you are stuck

I have tried Reunion,classmates, facebook ect. but not with the results I receive by the steps above!

Great! Thanks so much. I added your suggestion to our FAQs.

Here's another tip that's worked well. If the person has a common name, find someone who remembers that person's sibling or parent who has a LESS common name and search for them. My most dependable search engines? www.zabasearch.com/ followed closely by www.whitepages.com

Excellent. I just added that too.