This study describes a research focused on grammar school students' misconceptions in the context of common, everyday thermal phenomena. Almost 500 Czech grammar school students from 24 classes were asked to fill in the reduced version of the Thermal Concept Evaluation in order to measure the asset of traditional instruction in eliminating scientifically incorrect ideas; the reached normalized gain of 0.23 signalizes poor effectiveness of instruction as a whole. In some areas, Czech students' results were quite poor (questions dealing with phase transitions), while other ones turned out to be surprisingly good in comparison with foreign studies (e.g. perception of "cold" as a scientifically disproved concept). Besides questions regarding thermal phenomena, the test included four statements designed for studying possible relationships between students' scores and their attitudes. Those who declared "fondness for physics" showed better results both in the pre-test and the post-test; however, the normalized gain turned out to be independent of the attitudes established.