This study investigated the extensiveness of the procedures followed by science teachers in preparing SBA tools for science subjects in secondary schools in Kilimanjaro region, Tanzania. The reviewed literature indicated that ambiguity in following procedures for developing SBA tools resulted in to lack of assessment formative aspects in SBA practices, and hence there was a need of establishing a way forward. Guided by complex adaptive system theory, it was important to examine and explore the manner in which procedures for developing SBA tools were adopted and used by science teachers in enhancing teaching and learning processes in secondary schools in the region. This mixed method study adopted an embedded model of the convergent mixed research design whereby quantitative and qualitative data sets were collected concurrently. The target population involved science teachers teaching physics, chemistry and biology subjects, students, academic masters/mistresses, head masters/mistresses, district education officers and zonal quality assurers. Stratified, convenient and purposeful sampling techniques were used to determine the study sample that consisted of 913 respondents. Questionnaires, interview guides, interview schedule, observation guide and document analysis guide were used to collect the required information. The collected data were analyzed by descriptive statistics and inferentially using Chi-square and one way ANOVA techniques. Tables, charts, narrations and direct quotations were used to present the study findings. The study concluded that despite science teachers’ efforts, enthusiasm and diligence in integrating various techniques, there are no general procedures followed in order to develop and administer SBA tools formatively. However, SBA practices are comparable in public and private schools. The study recommended education stakeholders to device clear guidelines for development and administration of SBA tools in preparing SBA tasks.