on school day，例如：

1.On school day, what is the maximum speed through school zones in the city town or villages?在城市内上学日，在穿过校区地带时，最高的时速是多少？

2.Ironed shirts on a school day.在上学的日子穿熨烫整洁的衬衣。

3.On the school opening day, I rose up very early, earlier than usual, because I was greatly excited to see my new school.开学那一天，我很早起身，比平常时还早，因为我很兴奋的要看看我的新学校。

如果是说在学校的某一天，用：on a school day
如果表达通常的情况，用：on school days

应该是on school day，

因为在英语里，day前面加on。