Denver-based songwriter Mark Ledwig first penned Permanent Teeth as a classroom tool: An elementary-school teacher in Los Angeles, he knew his catchy numbers might help bilingual students comprehend such things as punctuation, the alphabet, multiplication and environmentalism. But after hauling some of his professional musician friends into the studio and recording under the name Natural Selection, Ledwig wound up with a recording that should appeal to fans of the Fab Four almost as much as the second-grade set. The CD is currently available exclusively through the Masterworks Music Services Web site, at mwms.net, but it’s definitely worth checking out from a music standpoint. As for the lyrics, who couldn’t use a little refresher in the basics?