I presume your instructor is using the rule: "hyphenate an open-form compound noun when it is used as an adjective preceding the noun it modifies." Not everybody agrees with this rule, but you can find it [online](http://owl.english.purdue.edu/owl/resource/576/01/).

From the above link:

> Use a hyphen to join two or more words serving as a single adjective before a noun:  
  - a one-way street  
- chocolate-covered peanuts  
- well-known author  
However, when compound modifiers come after a noun, they are not hyphenated:  
- The peanuts were chocolate covered.  
- The author was well known.

If you are writing for a publication where this is in the style guide, or taking a course where the instructor believes in this rule, you should probably follow it.