This study presents first-year findings of a 25-week longitudinal project derived from a two-year longitudinal randomized trial study at the elementary school level in Costa Rica on effective computer-assisted language learning (CALL) approaches in an English as a foreign language (EFL) setting. A pre-test–post-test experimental group design was implemented to evaluate two varying types of CALL curriculum (Treatment A and Treatment B, both technology based to assist English language learning) with a difference in students’ time-on-task, as opposed to the Control/Comparison group (that received no treatment and was a typical practice within the regular English teaching time period). Four subtests from Woodcock Munoz Language Survey-Revised (WMLS-R), a norm-referenced, standardized instrument, were selected to monitor participants’ oral English development. A total of 76 urban, rural, and urban/marginal schools with 816 third graders were included in the analysis through multilevel modeling. Results suggested that (1) students held very limited oral English proficiency at the beginning of the third grade; (2) although students significantly improved their oral English proficiency during the 25-week intervention, they were still significantly below the typical native English-speaking norm at the end of the third-grade level; (3) those who were exposed to CALL modules in Treatment A developed at a faster rate than did students in Treatment B and in Control classrooms in lexical knowledge and listening skills when statistically controlling for student-level variables, including initial level and time-on-task; (4) although students in CALL intervention (especially in Treatment B) started with a lower level of oral English proficiency, their gain was numerically higher than that in the Control condition; and (5) time-on-task demonstrated to be an irrelevant variable in the study. These findings imply that it is not just exposure to English that matters for significant gains in the language; rather, it is the type of instruction a student receives, or the quality of instruction in which the software engages the students interactively in a hands-on, minds-on, scaffolded manner, that matters the most in developing steep gains. Finally, we recommend that additional research be conducted with groups that move through kindergarten, first grade, and second grade longitudinally to determine cohort effects in learning English via CALL instruction in EFL countries.