I recently planned my high school reunion (this was number two!) and there are a few tips I though I'd share that made ours a success.
1) Give people lots of notice! I secured a date and started spreading the word in September for our June event. This gave people who were coming from out-of-town lots of time to plan.
2) Keep the ticket cost low and the event casual. I know of three sit-down dinner reunions that have been cancelled due to low attendance. Keeping it at a pub allowed people to come dressed like they want to, the opportunity to stay for as little or as long as they wanted and gave people the freedom to move about and chat.
3) Provide name tags. People do look different and even if you recognize their face, after 10, 20 or 30 years, you won't remember everyone's name. Go the extra mile and photocopy grad yearbook photos and attach that to the name tag.
4) Plan some element of surprise. I had a 90's themed candy bar, provided some fun 90's props plus a cool Instagram frame and at 10pm circulated mini cupcakes in school colours.
Overall it was a super fun night with many people suggesting another reunion sooner the later.
Best of luck with planning and if you want any details on the candy bar, ticketing, name tags or photo props, please send me an email. I'm happy to help.