In this work, we explore the use of digital technologies and statistical analysis to monitor how Italian secondary school students' writing changes over time and how comparisons can be made across different high school types. We analyzed more than 2,000 exam essays written by Italian high school students over 13 years and in five different school types. Four indicators of writing characteristics were considered--text length, text complexity, and two indicators of source use, all extracted using natural language processing tools--which provided insights into students' citation practices over time and in different school contexts. In particular, we measured the portion of students' essays that included text from source material as well as the amount of copied text that was not properly referenced. We found that student essays became shorter in length over time while also getting more complex. We also found that the tendency to copy uncited text in the essay decreased. High school curricula predict different writing strategies: essays written by students attending scientific and humanistic high schools are longer and less subject to incorrect citations. We argue that such text analysis enables the study of writing features in high school classes and supports the evaluation of curricula.