Try to obtain a complete list of your graduating class from your old high school. Some schools may also keep current contact information for any alumni who have inquired about upcoming reunions. If the school does not have a class list, use your high school yearbook as a reference. Once you are at the school, ask if they will put a link to your reunion website (from the high school website), and mention your reunion in any newsletters.
Once you have a master list of names, you can create a page on your website that has all the names of your classmates. It is recommended to have another section of that page (or another page) that has a list of the classmates that have been found. This can also be done on one list in different colours.