"第一天去上学"用英语可以说作 "the first day of school"。
"the first day of" 是一个短语，意思是 "第一天的"。而 "school" 则是 "学校"的意思。
下面是一些例句：
"I'm a pttle nervous about the first day of school." (我有点紧张第一天去上学。)
"The first day of school is always exciting." (第一天去上学总是令人兴奋。)
"I can't wait for the first day of school to begin." (我迫不及待第一天去上学。)