This topic clearly does not belong in "OS X Guests". The guest is Linux, but even that isn't very relevant. Moving it to "Using VirtualBox".
pachequin wrote:error: invalid arch-independent ELF magic.
Entering rescue mode ...
grub rescue
I dont know if this problem is related to windows or my VDI file.
I would guess that it's neither.
First, can we please emphasise that a VDI is a hard disk. You do not clone PCs by copying the hard disk, you must clone the entire VM, i.e. the motherboard description too. The fact that VDIs are large doesn't make them the only important file.
Ok, given that you have a clone of the VM, your problem is most likely that you have a grub boot script that references the boot drive using its UUID. This is quite common in Linux boot scripts. However the drive UUID changes when a clone is created, otherwise it would not be possible to mount the clone on the same host - it would conflict.
So, one solution is to modify the grub script so that the drive is identified by device name rather than id. I'm not au fait with Linux, but I believe the device name would look something like "/dev/sda".
Another possibility is not to clone the VM at all, simply copy the VM folder to the student PC. That way all UUIDs are retained and grub will be happy. This does mean that you can't mount the copied VM on your own PC, but I assume you really don't need to. See the related FAQ
Howto: Move a VM.