Having students play a few rounds of chess gets them anticipating what will happen next, which is exactly what the rebuttal of a debate is all about. This is a skill that is still developing for many middle school students, who tend to think impulsively without seeing how one action affects the next. Teaching students the valuable strategies in a chess match will serve them well for the most exciting part of the debate.
Advice for how to deliver the rebuttal portion of the debate, which can often be a chance for students to really shine with their educated arguments. They will need to quickly decide what points to abandon, and which to highlight in a short amount of time. They will need to strategize as a team to see which points will persuade the audience most convincingly.
This site has multiple lessons and PowerPoints on persuasive writing that are broken down by grade level to best suit your students' needs. This would ideally be presented, modeled, and practiced with students before you begin the debate unit.
An excellent list of potential debate topics that would work with middle school students. Many of these have a connection to science and the environment, as well as social studies. Great opportunity for cross-curricular development within a unit.
This is a debate template I created for use within my classroom with much success over the past few years. The debate unit also has students researching informational texts, identifying opinion from fact, pulling out central ideas, and creating persuasive writing that they then present during a real in-class debate.