Reunions are times to get back in touch with people you haven't seen in years. Whether a grammar school, high school, college, workplace, or any other type of reunion, these are great times to re-connect. Because you likely haven't seen many of these people in many year or won't see them for years to come, it's important that you leave them with a good impressions. To do so you need to dress well.
The dress code for a reunion should match the appropriate dress code for the place that the reunion is held (restaurant, bar, etc). At a minimum however, you should dress professionally in a business casual outfit. Lighter colored pants and shoes are not recommend, but darker pants and black shoes are suggested to give a more formal appearance. Like with any other business casual outfit, this should be accompanied by a long-sleeve dress shirts and matching belt.
Dress appropriately for the venue of the reunion
The nicer you dress, the better impression you will leave
Aim to dress at least as or more formal than the other attendees