Due to the predominance of test-oriented practice, one major challenge faced in Japanese high school classrooms is the lack of learning activities to develop higher order thinking. The purpose of this action research study was to explore an extracurricular blended learning program created to develop the higher order thinking of English language learners at a public high school in Japan. In this one-month program, 16 participants engaged in online synchronous and asynchronous activities, with English as a medium of instruction and communication, supported by in-person face-to-face sessions conducted in Japanese. Data were collected via asynchronous forums, a post-survey, and my observation notes. Results indicated that participants demonstrated higher order thinking to a certain extent overall in the forums; however, learner-learner interaction was not as highly activated as expected, mainly due to limited social interaction within the forums. These findings suggested that constructivist asynchronous forums can be used to develop the higher order thinking of English language learners in K-12 settings, with appropriate program design, instructor mediation, and content.