School stress is widespread among students' worldwide, impacting academic success and mental health. Most research focuses on average stress levels but lacks insights into temporal trends and inequalities. Using repeated longitudinal survey data on 33,000 students in Swedish compulsory school from the Evaluation Through Follow-up (ETF) study, we investigate temporal trends in stress across three cohorts of students (born 1992, 1998, and 2004), with a focus on inequalities by school year, sex, socio-economic status, migration background, school grades, and school difficulties. The results show that (1) stress increased more in year 6 than in year 9 in recent cohorts; (2) stress increased more for girls than for boys; and (3) low school grades and school difficulties have become stronger risk factors for stress. We discuss the findings in the context of recent educational reforms and broader societal trends concerning the role of education for young people's prospects in life.