Recording a class of 30 kids singing to a guitar track I laid down on Garage Band. Severe distortion because they have to hear the guitar track through a set of speakers..

I had a big setback today on a project I've been working on. I recently made my first recording of my own playing and singing, a guitar track and a vocal track of a children's song. When I recorded the guitar track, it took me a whole day to get it right. I used the metronome through earbuds. Then I listened to the guitar track and sang into a Snowball mike to get the vocal track down. Played separately or together, they came out crisp, clear, no dostortion. I made a CD of the song by sending the song to iTuines, with and without my voice so the children could practice to it. I taught the song to 30 fifth graders so I could add a vocal track of them singing. My plan is to use it as the music for a slideshow I'm making for their elementary graduation. I'm a mom, not their teacher. The teachers of the two classrooms have been generous in letting me teach the kids the song, and setting up 45 minutes for me to record it. Did the recording of the kids today at the school. Two tracks of all 30 kids doing their very best to cooperate. They sang well. I opened the garage Band file at home and it is such poor quality audio that it is UNUSABLE. There's a lot of fuzz and distortion. I can probably get their teachers to let me have them again to give it another try, but I better get it right. When I recorded my own voice to the guitar track, I monitored using earbuds. I can't hook 30 kids up to my Macbook with earbuds. I played the guitar track from speakers and held the microphone out toward them, so the mike was between the speakers and the kids. The speakers and mike both faced the kids. They sang really well, so it was pretty disappointing to find the recording sounded so bad. My guitar teacher tried to help me figure out what to do, but he thinks it is going to be very tricky to get it right since the kids have to hear the guitar track to sing it. I'm considering playing the CD I made of the guitar track from a boom box behind the kids and just recording the kids on a separate project file and losing the ability to edit, and probably losing most of the guitar quality and volume. But I won't know if it works until I have 30 kids and 2 teachers and all my gear set up to actually try it. Whatever I do, it has to work on my next try. There's very little time left and no budget. I thought my Macbook Pro and the Snowball, purchased at the Apple Store, would be enough.

Please advise. Thanks.