Technical schools are an integral part of the education system, and yet, little is known about student learning at such institutions. We consider whether assessments of student learning can be jointly administered to both university and technical school students. We examine whether differential test functioning may bias inferences regarding the relative performance of students in quantitative reasoning and critical reading. We apply item response theory models that allow for differences in response behavior as a function of school context. Items show small yet consistent differential functioning in favor of university students, especially for the quantitative reasoning test. These differences are shown to affect inferences regarding effect size differences between the university and technical students (effect sizes can fall by 44% in quantitative reasoning and 24% in critical reading). Differential test functioning influences the rank orderings of institutions by up to roughly 5 percentile points on average.