Can "City of students" be translated to "Da xue sheng cheng shi"?
For the first part, Wang Dingwei is right.

For the second sentence, guoqi(国旗) refers to the national flag. The city flag should be translated to "shiqi"(市旗). I think "shiqi de yanse shi huise he huangse"(市旗的颜色是灰色和黄色) should be right.