Worldwide Piano & Music School is where Bart teaches piano classes to Chris & Jerry in Seasons 7-9, and then Chris & Wyatt in Seasons 10-12. The place has actually been mentioned many times, in fact, Chris goes there often for piano lessons with Bart. However, this happens off-screen.
The interior is never shown, and the exterior is never shown, either, except for a picture of the place shown in Bart's Son, Part 2 in Season 10, and A Bigger Piano in Season 12. The name of the place was only revealed in this picture. If it wasn't for the picture the name (which is Worldwide Piano & Music School) wouldn't be revealed.
The place has a blue roof lining and white front. It has a parking lot, which could be just for the shop or the shop could be part of a strip mall, who knows. There are beige papers in the windows with indistinct words on them.
You can see this image in the gallery.
The exact location is not revealed but Chris and Bart mention multiple times that it is in Downtown Duesburg.
Bart runs the place and sells pianos and other instruments there. He runs piano classes there in which the only two known members are Chris and Jerry in Seasons 7-9, and Wyatt taking Jerry's place in Seasons 10-12. There are most likely other members in the classes but this information is unknown.
Until the episode Barryl's Back But Not for Harm in Season 9, the piano classes are actually held privately at Chris's house, because Chris had his own keyboard that they used. However, by that episode, Chris dismantled the keyboard and put it away in his closet due to lack of space. Bart got mad at this and actually kicked Chris out of the class for this. But after Jerry said no, Bart changed it so Chris is back in the class, but the classes will now be held at his shop in Downtown. And because Jerry lives closer to Downtown, Bart will pick Jerry up first and then Chris.
This further proves the point of other kids in the class, because Chris and Jerry had private classes but Chris and Wyatt didn't. However, as said, this information is unknown and thus, cannot be proven.
Technically, this is a gallery because it contains an image. It may be only one image, but it's an image.