Classrooms　are　high-risk　indoor　environments,　so　analysis　of　severe　acute　respiratory　syndrome　coronavirus　2　(SARS-CoV-2)　transmission　in　classrooms　is　important　for　determining　optimal　interventions.　Due　to　the　absence　of　human　behavior　data,　it　is　challenging　to　accurately　determine　virus　exposure　in　classrooms.　A　wearable　device　for　close　contact　behavior　detection　was　developed,　and　we　recorded　＞250,000　data　points　of　close　contact　behaviors　of　students　from　grades　1　to　12.　Combined　with　a　survey　on　students＇　behaviors,　we　analyzed　virus　transmission　in　classrooms.　Close　contact　rates　for　students　were　37　±　11%　during　classes　and　48　±　13%　during　breaks.　Students　in　lower　grades　had　higher　close　contact　rates　and　virus　transmission　potential.　The　long-range　airborne　transmission　route　is　dominant,　accounting　for　90　±　3.6%　and　75　±　7.7%　with　and　without　mask　wearing,　respectively.　During　breaks,　the　short-range　airborne　route　became　more　important,　contributing　48　±　3.1%　in　grades　1　to　9　(without　wearing　masks).　Ventilation　alone　cannot　always　meet　the　demands　of　COVID-19　control;　30　m3/h/person　is　suggested　as　the　threshold　outdoor　air　ventilation　rate　in　a　classroom.　This　study　provides　scientific　support　for　COVID-19　prevention　and　control　in　classrooms,　and　our　proposed　human　behavior　detection　and　analysis　methods　offer　a　powerful　tool　to　understand　virus　transmission　characteristics　and　can　be　employed　in　various　indoor　environments.　©　The　Author(s)　2023.　Published　by　Oxford　University　Press　on　behalf　of　National　Academy　of　Sciences.　This　is　an　Open　Access　article　distributed　under　the　terms　of　the　Creative　Commons　Attribution-NonCommercial-NoDerivs　licence　(https://creativecommons.org/licenses/by-nc-nd/4.0/),　which　permits　non-commercial　reproduction　and　distribution　of　the　work,　in　any　medium,　provided　the　original　work　is　not　altered　or　transformed　in　any　way,　and　that　the　work　is　properly　cited.