Normative rules of standard Serbian language are acquired during primary and secondary education through curriculum demands of Serbian language instruction, which takes place in three fields: grammar, orthography and culture of expression. Topic of interest in this paper is the quality of written expression of 6th and 7th grade pupils, in the context of all three fields specified to be mastered by the curriculum of Serbian language. Research comprised 148 primary school pupils from Belgrade. Linguistic analysis of spontaneously created written text was performed, in the conditions where it was not explicitly demanded form the pupil to write correctly. The results indicate that the majority of pupils make spelling and grammatical errors, meeting the condition for the basic level of mastering the knowledge in Serbian language according to the standards specified for the end of compulsory education. In addition to this, a considerable majority of pupils has a satisfactory level of culture of written expression. Pupils more often make spelling than grammatical errors. Seventh grade pupils are better than sixth grade pupils with respect to adhering to grammar rules and according to culture of written expression, while the mark in Serbian language and general school achievement of pupils correlate only with the degree of adhering to the orthographic rules. It was concluded that not only individual programs of support for pupils who make more errors are necessary, but also launching national projects for the development of linguistic competence of the young in Serbia.