Response and more questions
Every hard drive is formatted to a particular file system. For instance, there are main types in the realm of Windows and Mac (NTFS, FAT,HFS). Mac uses HFS and FAT, but not NTFS. Windows can read NTFS and FAT, but not HFS.
This is one of the principal differences between Mac and Windows. NTFS formatting will not read on any Mac(without some emulation or application). If your students have a flash drive formatted by a PC, chances are that it is possible they formatted as NTFS (NTFS is file system that was created for better security and can pretty much only be read off windows)
Basically, if they do have this "NTFS" formatting. All you would need to do is to tell those students to reformat under FAT and then save their files on the drive. Then you should be able to read the files properly.
If they are using a PC, to check whether the file system is FAT or NTFS, tell them to go to My computer, right click the flash drive and click on properties. That should show which file system that the drive is using. If they are using NTFS, tell them to backup the files onto the computer hard drive and reformat the flash drive using FAT. This will wipe out all the data and create a FAT file system format. Then have them save the files on the flash drive again and give them to you.
That should solve the problem if they are using NTFS