There are many rules of the school 和There are many rules at

of the school （属于）这所学校的
at school 在上课,在上学；在学校
There are many rules of the school
这所学校有很多规定.
There are many rules at school .
这所学校有很多关于学生/上学/在学校的规定.
祝你开心如意!