Over the past several years, research on school improvement and professional development has identified a consistent message in regard to the most effective ways to improve student achievement. Collaborative Literacy Intervention Program (CLIP) can be regarded as a more effective way to reach students who were having difficulty in reading. By examining average classroom test scores, this study provides some quantitative evidence regarding the use of CLIP as a professional development tool. The quantitative analysis includes data from two classroom teachers in Tehran, Iran. First, teachers self-selected to be involved in the CLIP training and each teacher had an equal opportunity to participate. Then, four classes were selected as the participants. In general, 65 students in all four classes taught by the two teachers selected for the purpose of this study. Based on PET proficiency test, those placed between one standard deviation above and below the mean were selected. Finally, 38 students were selected as the main participants, 20 for the experimental group taught based on CLIP and 18 for the control group. The TOEFL proficiency test was used as the pretest and posttest for all selected participants. Then, average classroom test scores of students in the class taught based on CLIP training were compared with the class taught without using CLIP training. The results of data analysis indicate that there was a statistically significant difference between the students’ performance in experimental and control group in the results of pretest and posttest. In other words, CLIP training played a significant role in developing the students’ performance in reading, grammar and vocabulary. In conclusion, this study showed that training teachers through CLIP is a suitable pedagogical approach to promote language skills.