This study examined the nature and frequency of error in high school native English speaker (L1) and English learner (L2) writing. Four main research questions were addressed: Are there significant differences in students' error rates in English language arts (ELA) and social studies? Do the most common errors made by students differ in ELA and social studies? Are there significant differences in the error rates between L1 and L2 students in ELA? Do L1 and L2 students differ in how frequently they make the most common errors in ELA? Written work of 10th and 12th grade students in five states was collected. The sample included 178 essays (120 in ELA and 58 in social studies) from 67 students (33 10th graders and 34 12th graders; 49 native English speaking students and 18 English learners). Results indicate that there were significant differences in the frequencies of errors between ELA and social studies, with higher error rates in social studies. In addition, L2 writers had significantly higher error rates than L1 writers in ELA. Aside from a few types of errors (spelling, capitalization, and some punctuation errors), most types of errors appear relatively infrequently in school-sponsored writing. Moreover, the eight most common errors accounted for a little more than half of all errors, and these did not differ significantly between ELA and social studies writing or between L1 and L2 writers.