不得穿校服用英语可以说成 "not allowed to wear school uniform"。

"Not allowed" 这个短语的意思是 "不允许"，"to wear" 是动词 "wear" 的不定式形式，意思是 "穿"，"school uniform" 是 "校服" 的意思。

下面是一些使用 "not allowed to wear school uniform" 的英文例句及其中文翻译：

Students are not allowed to wear school uniform on weekends. (周末学生不得穿校服。)
The school has a strict dress code and students are not allowed to wear school uniform outside of school hours. (学校有严格的着装规定，学生在校外不得穿校服。)
During summer vacation, students are not allowed to wear school uniform. (暑假期间，学生不得穿校服。)