Teaching primary reading and writing is a specific area of teaching the Croatian language. Accordingly, it has special requirements and principles of teaching. Teaching writing is based on the standard school orthography, introduced by the Croatian Spelling Dictionary (2013), the use of which is mandatory in all student materials (textbooks, workbooks, spelling books, notebooks and other materials). The purpose of the paper The Application of Standard School Letters in Primary Education is to examine the extent to which pupils of fourth grade primary schools from the Republic of Croatia deviate from the orthography, taking into account each single letter and the specific connections of certain letters. Apart from that, the paper also examines whether the texts of the students have been violated legibly. The statistically significant differences were determined according to the sex of the pupils and the working life of their teachers. The results have shown that boys diverge statistically significantly more both in writing certain letters and in connecting letters. Pupils taught by teachers with more work experience are generally less diverging from the standard orthography than the pupils taught by teachers with less work experience. In most cases, the readability of the written student work is not violated.