Many English-as-Second-Language (ESL) learners find it highly challenging to write problem-solution essays. This difficulty is partly caused by the pedagogies commonly used in traditional classroom settings, which have two major in-vivo constraints: time limits and low student engagement. This study proposes an innovative theory-driven instructional model for teaching problem-solution writing, namely the flipped 5E PSW (problem-solution writing) model. The flipped 5E PSW model is built upon three theoretical or conceptual models: (a) Jonassen's design theory for case/policy analysis problem-solving, (b) the flipped learning model, and (c) Bybee's 5E learning model. Two groups of 23 ESL secondary school students, both taught by the same teacher, were assigned to either the flipped or non-flipped versions of the instructional model. The students were assessed individually over a two-year longitudinal experiment to measure the impact of the intervention. The results, as measured by mixed ANOVAs, indicated that the flipped 5E PSW model was more effective than the non-flipped version for improving students' performance in problem-solution writing. This application of the flipped 5E PSW model in a two-year real-world school environment has demonstrated its capacity for overcoming traditional classroom constraints.