This paper presents the results of research conducted with the primary objective to determine in which areas secondary school students usually make orthographic mistakes when writing (official) written assignments. Starting from the hypothesis that the punctuation writing of whole and split words are areas in which secondary school students (regardless of age and school orientation) achieved the weakest achievements an (exploratory) research was conducted on a corpus of 3,135 written assignments written in the school year of 2010/11. The research sample was intentional, descriptive and analytical methods were used for the description and the analysis of the results. The results showed the following (1) secondary school students usually make mistakes in punctuation of written assignments - we recorded 4,487 errors in the use of signs to denote intonation and meaning of a text (errors of this type make 53.93% of the total number of spelling errors reported in the corpus of research); by frequency of errors the second are errors related to writing whole and split words (11.02%), the third error is in the use of the capital letter (9.34%); (2) most problems in orthography have second grade students, quantum of mistakes is almost the same with first graders and seniors, but in all grades the most frequent errors are in punctuation, writing of whole and split words and the use of capital letters; (3) Although school orientation affects the spelling skills of pupils, the weakest orthographic achievements are also recorded in punctuation, writing of whole and split words and capitalization, so those are areas that need to be thoroughly addressed in teaching and methodology literature. The results are, on the one hand, a picture of the current status of teaching orthography and grammar knowledge of secondary school students. On the other hand, the research results can be applied in all phases of methodical practical work in teaching orthography, the upgrading the curriculum, in defining the goals and tasks of teaching languages, as well as in the selection of orthographic contents. At the same time, they constitute a strong impetus for further research on the path of modernization of teaching orthography.