Agree with most of the above. Ditch the backdrop except for headshots; shoot against the brick wall and on the wooden stage if that's their usual venue. More importantly, stand farther away, use a longer focal length, and shoot from waist-height to minimize or eliminate distortion. You've got plenty of room to work in that venue, so use it.
For some of the more skilled dancers, get some action shots. Use a single bare light to freeze them mid-leap and cast an interesting shadow on the wall. Or, a soft light from the front and a hard edge light from the back.
Alternate group shot: Have the dancers stand in a line on the edge of the stage facing the audience. Put the camera on the floor at the back of the stage, put a single bare Speedlight high up in the audience seats. Use a wide-angle if you must to get all the people in the shot. You'll get a cool group silhouette with long shadows thrown onto the stage.
Personal nitpick? Make sure the camera sensor is perfectly parallel to the wall. In your group photo, see how the bricks at the top are angled down, even though the "horizon" at the back of the stage is level. Take a step to the right and turn the camera a smidge left, to eliminate this effect. Standing farther away will make things easier.
I went above-and-beyond with the portraits for my wife's dance group, building a big backdrop and set and lots of props for their portrait session early this summer. Then in October, due to space/time/logistics constraints, I did portraits of the Rocky Horror cast against the brick wall in the alley behind the theatre. Both worked, for different reasons.
... I'll see if I can post some of those when I get home, to give you some ideas. My lighting and editing style might not be appropriate for a kids' dance studio, though.