Each time you sit down to work on a school computer, you will need to make sure Dreamweaver recognizes the site folder you are working on.
First, go to Site >> New Site
The Site Setup Window pops up:
Name your site, then click on the little folder by Local Site Folder to find the folder you have already created or create a new one. Make sure this folder is on the desktop.
Once you have filled in the Site Setup window, click save and Dreamweaver will recognize your folder.