Although a mass of studies has demonstrated the effectiveness of scaffolding through group-dynamic assessment (G-DA) in fostering English as a foreign language (EFL) learners' language skills and components, its contributions to developing psychological constructs, such as motivation, learning anxiety (LA), and willingness to communicate (WTC) have remained largely unexplored in the context of Iranian high schools. Thus, this study purported to disclose the contributions of G-DA to Iranian high school students' motivation, LA, and WTC. For these purposes, 124 grade 11 students were chosen through a random sampling method at Shahed High School in Borujerd City, Iran, homogenized through the Oxford Quick Placement test (OQPT), and randomly allocated as a control group (CG) (n = 23) and an experimental group (EG) (n = 23). Afterward, a pre-test, interventions (for 16 one-hour sessions held two times a week as an extra-curricular program), and a post-test were implemented. The results of the independent samples t-tests evidenced that EG's motivation significantly improved compared to CG after the interventions. Additionally, the findings revealed that the G-DA-based instruction significantly contributed to relieving EG's LA. Further, the results uncovered that a statistically significant difference existed between EG and CG concerning the gains in WTC. The findings gained in this study are anticipated to contribute significantly to the various EFL settings, as well as they offer a variety of recommendations and implications for relevant stakeholders.