1. Go to the folder  of interest in the Document
Library.
2. In the search  picklist at the top of that
folder, select "this folder".
3. Enter the search word or phrase in the "for"
box.

Thanks, I actually wanted to 'deny' access to a
specific folder  from the main portal search. And If
someone wants to search  on this folder, he can go to
this folder and search only here ?

Is there a way to do this ?

There is another solution for your search.
You can create a content  source who indexes only the folder  of your project
school. And in the tab advanced you can create a search  scope. You write the
name of search scope and now, when you go to the search sub-dahsboard, you
have the option this site for the search and now you have your search scope
too.