This study sought to find out the impact of Senior School Science Students having mathematics lessons three times a week as against the usual five times a week in the school time – table in Nigeria. The research design adopted was PreTest-post-test Quasi-experimental. The population of the study was 1,092 Senior School Three (SS3) Science Student from 26 public Senior Schools from Oshimili North and South Local Government Areas of Delta State, Nigeria. The sample of the study was 252 Schools Science Students in Oshimili North and South local government Areas of Delta State, Nigeria. Two research questions and one null hypothesis were posted to guide the study. A validated and reliable (0.85) instrument, Mathematic Assessment Test item (MATI), was used to collect data, which was analyzed with z-test statistic. It was found that students who had mathematics lesson five times a week in the time-table had higher Mean Percentage Score (76.1) than those who had mathematics lesson three times a week in the school time-table with a Mean Percentage Score of 74.7. However, the difference in Mean Percentage Score was not, statistically, significant. It was, therefore, among other things recommended that the school principals can use the time-table of three mathematics periods a week, with adequate assignments for science based students, at the senior school Level.