Dynamic assessment (DA) is believed to have potentiality to affect EFL learners’ language development; however, its different forms might be of varying types and size of washback effects on foreign language development. So far, it seems few studies have compared the effects of interactionist and interventionist models of DA on the grammar learning of EFL learners; moreover, whether DA procedures have different impacts on the learners in the contexts of official high schools and private language institutes has received scant attention. Against this backdrop, 96 intermediate EFL learners from a high school and a language institute were put into experimental groups of A and B and a single control group (C). During 12 treatment sessions, group A participants were taught and assessed through interactionist DA procedures, group B received interventionist DA practice and group C was assessed through conventional multiple choice assessment procedure. The results showed that both interactionist and interventionist DA practices had significant effects on grammar learning of EFL learners. However, interactionist DA was more effective in language institutes with limited number of learners while interventionist approach was of higher impacts in high schools with larger groups of learners. The findings imply that the validity of different DA forms is sensitive to the practicality of the procedure in the assessment context.