Extremism continues to raise concerns about conflict and violent attacks that can lead to deaths, injuries, trauma, and stress. Adolescents are especially vulnerable to radicalization by extremists. Given its location in a region that often experiences extremism, Bahrain developed 4 peaceful coexistence lessons and 4 antiextremism lessons to be implemented as part of their Drug Abuse Resistance Education (D.A.R.E.) program. The aim of this study is to report the results of the preparation phase of the multiphase optimization strategy (MOST) to develop a peaceful coexistence program and an antiextremism program implemented by D.A.R.E. officers in Bahrain. We developed conceptual models for the peaceful coexistence and antiextremism programs, indicating which mediators each lesson should target, the proximal outcomes that should be shaped by these mediators, and the distal and ultimate outcomes that the intervention should change. We recruited 20 middle schools to pilot test our research protocol, survey measures, and the existing intervention lessons. A total of 854 seventh and ninth grade students completed a pretest survey, 4 peaceful coexistence intervention lessons, and an immediate posttest survey; and a total of 495 ninth grade students completed the pretest survey, 4 antiextremism lessons, and an immediate posttest survey. A series of 3-level models, nesting students within classrooms within schools, tested mean differences from pretest to posttest. Pilot test results indicated that most measures had adequate reliability and provided promising evidence that the existing lessons could change some of the targeted mediators and proximal outcomes. Specifically, students who completed the peaceful coexistence lessons reported significant changes in 5 targeted mediating variables (eg, injunctive norms about intolerance, P<.001) and 3 proximal outcomes [eg, social skills empathy (P=.008); tolerance beliefs (P=.041)]. Students who completed the antiextremism lessons reported significant changes in 3 targeted mediators [eg, self-efficacy to use resistance skills themselves (P<.001)], and 1 proximal outcome (ie, social skills empathy, P<.001). An effective antiextremism program has the potential to protect youth from radicalization and increase peaceful coexistence. We used the preparation phase of MOST to (1) develop a conceptual model, (2) identify the 4 lessons in each program as the components we will evaluate in the optimization phase of MOST, (3) pilot test the existing lessons, our newly developed measures, and research protocol, and (4) determine that our optimization objective will be all effective components. We will use these results to revise the existing lessons and conduct optimization trials to evaluate the efficacy of the individual lessons.