Despite growing interest in the studies on Zone of Proximal Development (ZPD), its operation in the forms of individualized and group-wide has been controversial. To cast some empirical light on the issue, this study was designed to study the applicability of the two scenarios of ZPD-based instructions to the writing accuracy of two levels of Iranian EFL learners (low vs. high scorers). To this end, 118 EFL learners identified as homogeneous based on the TOEFL ITP test were randomly assigned into two equal experimental groups which, respectively, received compatible ZPD-based instructions (i.e., individualized--I--and group-wide--GW) utilizing conference writing (as an individual scaffolding technique) and structured writing templates (as a whole-class scaffolding technique). The treatments were rendered to the groups in a random manner within homogeneous blocks. Before and after the treatments, three tests (two writing tests and one ZPD test) were administered to measure both groups' writing abilities (i.e., accuracy) and their ZPD levels. A ZPD test was also administered in the mid of treatments. The non-parametric analysis of covariance (ANCOVA) indicated the usefulness of I-ZPD-based instruction to the writing accuracy of the low-scorers and the uselessness of group-wide ZPD-based instruction to the writing accuracy of both the low and high scorers. The findings yield support to the use of ZPD-based instructions in TEFL writing and thereby affecting the learners' writing accuracy. The study; indeed, yield support to the feasibility of GW-ZPD measurement along with I-ZPD approach.