A teacher-researcher searched for and defined theoretical principles in the literature that could be used as a basis for teaching high-school students who were struggling in mathematics (in a regular-level mathematics classroom) and could offer a framework to design tasks to encourage a problem-solving, thinking classroom. After identifying 13 principles, she refined them to a list of nine. Over the duration of the academic year, she designed lessons for two high-school classes (one Grade 11 and one Grade 12; 300 lessons in total) according to the principles to promote her class's group problem-solving skills, carefully documented them all, and then analysed her findings. The principles identified pertain mainly to awareness of how to promote active thinking and meta-cognition, explicit heuristic strategies for problem-solving, and using worked examples. All these to allow students (not only struggling) to build a general schema for approaching problem-solving with increasing efficiency. Two authentic class scenarios demonstrate how specific principles were applied and their effect on problem-solving efficiency in the classes. The findings suggest that introducing these principles into the class culture can offer a significant way to improve group problem-solving skills and, in particular, involve struggling students in the class discussions.