Cameo Appearance

Found this image attached to a mail entitled "カメオ出演。確認してください" (cameo appearance image - please confirm) in my inbox just now. Looks like Mirai with trumpet, Kanata with violin and Haruka with xylophone mallets. Not sure whats going on but I kinda like it so uploaded the 2560px x 1600px version to Flickr.

Also found this attached to the mail too. Would be grateful if somebody could let me know whats going on.