This is a quasi-experimental study that investigated the effect of collaborative learning, reteaching, and collaborative learning-reteaching as follow-up strategies to the mastery level of 79 academically challenged third high school students using quantitative and qualitative research methodologies. The distinctive feature of this design was the random assignment of subjects to collaborative learning, reteaching , and collaborative learning-reteaching as follow-up strategies. This study was conducted to ascertain the follow-up strategies that could most effectively enhance the mastery level of academically challenged students in chemistry. The study was conducted in a public secondary national high school during the third quarter of school year 2013-2014. A total of 79 academically challenged students of the school were used as subjects. The subjects belonged to three intact sections that were randomly assigned to collaborative learning , reteaching, and collaborative learning-reteaching follow-up strategies. A 35 item multiple choice, researcher made pre-test was administered to the subject prior to the interventions and the scores obtained were used as indicators of the subjects' prior knowledge about the selected topic (periodic Table of Elements). After the intervention, a post-test was administered and the subjects' posttest scores were used as bases to describe their mastery level. Qualitative data were also collected through interview, journal entries, and the obervers' observation notes, which were triangulated to arrive at valid results and findings. Quantitative data, on the other hand, were analyzed statistically using mean and standard deviation for descriptive analyses and t-test for dependent sample and One-way ANOVA, for inferential analyses. The four null hypotheses were tested at the significance level alpha = 0.05, test. Results showed that based on posttest scores, the three groups of the subjects had average level of prior knowledge of the topic Periodic Table of Elements which did not differ significantly from each other. In terms of their mastery level based on pretest scores, the subjects exposed to reteaching follow-up strategy registered a high mastery level while the other two groups, collaborative learning and collaborative learning-reteaching follow-up strategies had average level. The differences, however, were not statistically significant at 0.05 level. The difference between pretest and posttest scores, mean gain, of reteaching group was significantly higher than that of collaborative learning and collaborative learning-reteaching groups. On the qualitative aspect, the "stories" of the subjects generally related to the positive effects that the follow-up strategies had on their knowledge, attitudes, like cooperation, sharing and helping, and skills, like communication, listening, and social skills.