The mathematics teacher’s ability to assess and integrate assessment data into instructions is a function of their assessment literacy. While the teacher’s assessment literacy level is a determinant of quality of mathematics education delivery, research in the domain is limited. The study employed a descriptive survey design to explore perceived levels of assessment literacy of mathematics teachers in implementing classroom assessment within the school based assessment framework. A 35-item classroom assessment inventory was adopted, modified and administered to a purposive sample of 96 mathematics teachers from eleven Senior High Schools in Ghana. Seven other items were included to further collect data on their self-efficacy beliefs about their assessment literacy. Data were analyzed using descriptive statistics, Mann Whitney U test and correlation analysis. The result shows that majority of the participants did not attain the required assessment literacy standards for quality assessment in mathematics classroom. Moreover, there was a mismatch between participants’ assessment literacy and their self-efficacy beliefs in assessment in mathematics education even when gender and teacher experience were controlled. The study recommends that institutions training mathematics teachers or providing professional development should accentuate assessment literacy development to ensure standard assessment practices and quality instruction in mathematics at senior high schools. The assessment inventory used in this study could be adapted for use in college of education in Ghana to measure mathematics teachers’ literacies in assessing students.