小明今天不上学用英语可以说 "Xiao Ming is absent from school today."
"Absent" 是英语中表示"缺席"、"没有到场"的词汇。它来源于拉丁文 "absentem"，意思是 "离开"。
例句：
"Xiao Ming is absent from class today." (小明今天缺课)
"The teacher marked John absent for the day." (老师把约翰标记为今天缺席)
"She was absent from work yesterday." (她昨天没有上班)