According to their blog post (in Japanese!),
they got punished as a "penalty game" in "ヤツルギ4周年&映画決起大会&ポリオワクチンチャリティーイベント"(lit.the 4th anniversary of "Yatsurugi" and the rally and the charity event for the Polio vaccine) at T-Joy Soga.
Although i tried to find those movies, but i can not get...
But we might watch the event itself on youtube,
PS. I found spanking scenes on youtube.
Girls get punished at 08:13,17:06,17:50,25:00,30:00...