Many industries implement online learning for training and professional development to enhance the student experience. The specific problem addressed in this study was school staff do not have adequate knowledge of mitigating violence, such as school shootings, with ineffective current professional development. This design and development model use case study aimed to use a validated instructional design model to design, develop, and implement professional development for K-12 teachers and staff to mitigate school shooting risks as informed by content experts. The conceptual framework for this study consisted of constructivism and interpretivism theories for adult students. The student experience was assessed with Kirkpatrick's evaluation Levels 1 and 2. The samples for this qualitative study included three content experts and four K-12 teachers. Data were analyzed using deductive and inductive coding and Clarke and Braun's six steps of thematic analysis after coding using NVivo. K--12 teachers found value in the content and delivery of the improved professional development and feel better prepared to assist troubled students and prevent violence. The teachers expressed positive feedback about continuing such training and performed well on the summative assessment. Study implications are using a validated instructional design model and panel of experts to improve the student experience and professional development content improves preparedness. Sensitive content can be effectively delivered using instructional design best practices. Participants feel better prepared to work with others in the community, such as law enforcement, to improve training. This study contributes to practice by demonstrating how professional development about school shootings can be relevant and engaging. Social-Emotional Learning programs and practices should expand, and K-12 teachers and staff must foster partnerships with local law enforcement and other emergency management experts. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]