Online instruction can expand access to education for disadvantaged groups, yet it often deepens performance gaps. We study its impact on high-stakes exam outcomes using administrative data on five cohorts of students in 31 Israeli vocational colleges and the abrupt shift to online instruction during the COVID-19 pandemic. In this setting, exams were held in person and graded centrally, ensuring comparability to pre-pandemic performance. A difference-in-differences design comparing outcomes within students and across cohorts shows significant declines in exam attendance and demonstrated knowledge after the switch to online instruction. These effects are not explained by local infection rates or childcare responsibilities and are especially pronounced among Arabic-speaking minority students, regardless of socioeconomic status. Drawing on variation in infrastructure, residential crowdedness, language of instruction, and prior academic performance we identify poor internet access as a key mechanism. In addition, while the negative effects on majority students are concentrated among lower-performing students, for minority students the effects are, if anything, larger among high achievers.