Update you are replying to:
Yes this point niggled all the time I was watching the movie but as I now find it hard to catch all the dialogue I assumed I was missing something that would elucidate the point: just what motivated the two girls to put up with it.
As you say, they had to all intents and purposes left the school. Unless the school still had some hold on them the episode presents as consensual play.
But were you really trying for a straight face? Had you not told us I would not have thought so. Arguably, the Gilbert-and-Sullivan-like quirkiness is requisite and that might not have been realised but for the happy accident of your not quite straight face complementing the girls' gleeful ones..