This corpus-based study investigates lexical richness in English writing by Chinese senior high school students. Lexical uses in 303 compositions were compared across three grades in terms of lexical sophistication, variation, density and errors. Timed compositions were sampled from Writing Corpus of English Learners, and the sample sizes of three grades remained almost the same. The data were analysed through Range and Lexical Complexity Analyser to compute the four measures. In general, it is found that these students used a limited range of vocabulary and committed a number of lexical errors. In particular, one-way ANOVAs, Kruskal-Wallis tests, and post-hoc analyses showed that the growth of sophisticated word ratio accelerated in the first two years but decelerated in the third year. Lexical variation gained a significant increase across grade levels. Lexical word ratio decreased remarkably but increased slightly during the three-year learning. There were non-significant decreases in lexical errors between adjacent grade levels. These lexical features can be largely attributed to students' insufficient mastery of cohesive devices, improper English instruction in classes and pressure of the National College Entrance Examination. Based on these findings, we provide suggestions for foreign language teaching in senior high schools in China and other educational contexts.