This article explores the development of sentence complexity in the writing of young learners of Spanish as the third language (L3) learned in Norwegian schools. Learning to write in a foreign language is a complex process, and a novice learner will start to write simple sentences that develop over time. To track this development, the present study investigates texts written by learners of Spanish from the first and second year of upper secondary school (school years 11-12, ages 16-17) as their ordinary schoolwork. The texts are collected as part of the TRAWL (Tracking Written Learner Language) corpus and show how learners write in an authentic school context. To address sentence development, the use of coordinating and subordinating conjunctions is analyzed. The findings indicate that most pupils use y (and) and pero (but) to coordinate clauses. In subordinate clauses, que is frequently used to introduce relative clauses and porque to introduce causal clauses. There are only a few examples of a more varied use of conjunctions, indicating individual differences in the process of developing sentence complexity. The studied features are estimated to be acquired at the A1 level in the Common European Framework of Reference for Languages (CEFR), while the pupils in this study are expected to be at the A2 level. The findings show that there may be a discrepancy between the pupils’ actual knowledge and the expected knowledge according to the reference levels.