This study was conducted at a college in Ontario with a sample of students from the Game Development program for 5 intake years: 2014-2018, with the purpose of determining the validity of the Mathematics Diagnostic Test used to place 1st semester college students directly into an advanced math course (Math 10), or to leave them in the introductory math course (Math 9). Two streams in senior high school were considered, college preparation and university preparation, for a correlation study between students' grades in senior math courses and GPA, and their math diagnostic test scores. Next, a multiple regression analysis was employed to find any significant predictors of students' prior knowledge to the math diagnostic test. Further, a "t"-test for independent groups was used to determine whether students' placement in the two college math courses was valid. The study found a medium correlation between two senior college math courses (MBF3C and MAP4C) and the math diagnostic test. The multiple regression analysis established that they explained 18.5% of the variability of the diagnostic test. For the students in the university-preparation stream, there was a higher correlation between the senior math courses MCR3U and MHF4U and the math diagnostic test: they explained 44.1% of the variability of the diagnostic test in the regression model, which is a strong size effect. Further, the linear regression showed that the GPA explained 9.0% of the variability of the diagnostic test, which is considered a weak size effect. Finally, the "t"-test for independent samples found that there was not a statistically significant mean difference in Math 10 grades between the two groups: students with scores above 85% in the math diagnostic test placed directly into Math 10, and students with scores below 85% who were directed to Math 9 course first, then into Math 10. Therefore, taking Math 9 before Math 10 helped the weaker and regular knowledge students raise their knowledge to a level similar to the level of knowledge of students going directly into Math 10. The validity of the math diagnostic test to show students' prior knowledge in math was confirmed.