When To Start
Start planning any where from 2 years to 6 months, depending on how elaborate you want your event to be.   If your planning an informal event, limited food and entertainment then you can plan it in 6 months. Usually this is for 5 year reunions. Find a venuel, order finger foods, get a DJ and put up a few decorations.  If your planning a sit down dinner, or your classmates are not in the immediate area, plan on a year or more of lead time. Most reunions are during Thanksgiving weekend. That means every high school in the area may be looking at similar venues. The more formal the reunion the more lead time you'll need to book your venue.