There should be NO issues with opening a ST6 project using ST7... As far as I know the Student addition does not work differently then the full version, regarding releases. ST7 is fully backward compatible
Not sure why you're having a problem with this.
Can you post the Model/Aseembly so others can view and see what's up?
As Bob stated you can always open older versions with the new version of Solid Edge, however there are limitations of what Solid Edge can open based on the license types used to create the file versus the license type being used to open the file. For instance, a file created with a Student license cannot be opened in Solid Edge using a Commercial license.
Essentially it is designed this way to discourage folks from using free learning/hobby versions to perform commercial work.
When I open a project that I have done with ST6 and I open it with ST7 it work, But the probleme is when I do something with ST7 and after I open it with ST6, there is like a little pop up that open and it's write something like : you can't open this.
(I transfert my project with a USB key)
That is correct. You can only open files in the version they were saved in or a newer version. Once they are saved in ST7, they can no longer be opened in ST6 or any previous version.