The situation began with a modest object — the thermometer in a classroom. In June, temperatures exceeded 35 degrees Celsius in many schools across France, leading to the closure of 1,800 schools during a heatwave on June 23. Three months later, students who had endured stifling conditions behind blocked windows took to the streets. The movement began in late September at a private high school in Créteil, where three teachers, an assistant principal, a social worker, and a psychologist joined forces. By October 1, the Ministry of the Interior reported that 1,027 high schools were affected, with 222 of them blocked. On October 6, 891 schools experienced partial or full class suspensions, and 488 people were detained. More than 6,000 people have been arrested since the beginning, mostly minors. The organizers claim 450,000 demonstrators. The government has struggled to balance condemning violent groups with opening a consultation platform. Receiving representatives from high schools on October 2 did not suffice to calm the growing frustration. These stifling classrooms owe nothing to chance. Between 1964 and 1969, 1,150 secondary schools were built, as recalled by the Senate in 2023. Designed with concrete and large glass windows, this architecture was intended for the post-war baby boom generation, not for summers with temperatures reaching 40 degrees Celsius. According to surveys by Snes-FSU, half of the schools have no external solar protection. The cost of renovating 60,000 schools, colleges, and high schools is estimated at nearly 40 billion euros by the Banque des Territoires. The EduRénov plan provides only a fraction of that. The green fund, the main financial tool for such projects, dropped from 2.5 billion euros in 2024 to 800 million this year, with an expected rise to one billion in 2027. High schools fall under regional authorities, whose efforts vary widely — from 2,380 euros per student in Île-de-France to 5,830 in French Guiana. The thermometer also reflects the broader economic landscape. In constrained budgets, building maintenance is often an adjustment variable. A wall does not protest — until the students who occupy it do so in its place. The paradox lies in the National Education budget, which is not sacrificed but instead increases by 1.1 billion euros in 2027, reaching 65.5 billion euros. However, this budget mainly funds salaries, and demographics dictate the direction. With 180,000 fewer students expected, 1,588 teaching positions will disappear. This calculation holds at the national level but not in a class of 35 students without a French teacher. Public debt remains a silent but omnipresent factor. By the end of June, public debt reached 3,595 billion euros, or 119% of GDP, according to Insee. The French Treasury Agency expects to borrow a record 340 billion euros next year. The interest burden would reach 91 billion euros in 2027, according to the finance bill. Less than six months of interest would be enough to renovate the entire school infrastructure. For these teenagers, the equation is clear — they inherit the concrete of the 1960s and the financial burden of the 2020s. In 1990, a similar movement that began in a Parisian suburb secured an emergency plan of 4.5 billion francs. The state could then borrow to invest. The current situation must first reassure creditors, less than seven months before the presidential election. The autumn break may end the blockades, but it will not lower the thermometer. The next heatwave will raise the question again: who will pay for the adaptation of the school system — those who incurred the debt or those who will inherit it?