Hello, I am a music teacher. I have been emailing my students an email with a piano and vocal recording (on the same mp3). Most of the time, the way it works is that they download the email into Garage Band file Transfer, put the mp3 into GB, and then they have the karaoke track they're supposed to sing along to. Today, I had a student who said that when he listened to the file in his email, it sounded right, but then when he put it in GB, it played only the piano part of the recording. How is this even possible?

If I understand correctly what you're sending them is just a single audio file that includes piano and vocal parts within the same audio file. If there's only one track in the GB Project, and it's an Audio track with a single Region for that audiofile with both the piano and the vocal, then I don't see how it would be possible to be only hearing only the piano portion of that single audio file. But if there's a separate piano track in GB (Audio or Software Instrument) then it could be that the Instrument track is playing but the Audio track is not.