School assessment is a determining factor for a pupil in all functions. The aim of the research was to determine correlation between pupils' logical thinking and their school assessment and between pupils' mathematics skills and their school assessment. The sample size was created by 252 high school students (117 boys and 135 girls). As research tools were used reasoning mathematical, logical thinking and mathematical skills tests. Kruskal-Wallis test was used as statistical method. A significant effect was found at the 1% significance level between these factors and school assessment, where the greatest differences are caused mainly by evaluations in mathematics that were perceived either as the best or the worst. The research also monitored the use of the newly-developed GTOLT (Group Test of Logical Thinking), which is based on the GALT (Group Assessment of Logical Thinking) and TOLT (Test of Logical Thinking) tests, rooted in standardized tools.