[steven]

100% mocking. I have heard it said the exact way you are describing. It's a combination of kids (or some times adults) who like to say the phrase 'pera pera' and them trying a little bit to make fun of you (and possibly the teacher who said it to begin with). Don't let it get to you. The teacher probably won't do anything about it anyways.

In my opinion there are a couple of options:

1. Play along, say something in a terrible accent on purpose, and get more laughs than the kid. (in a weird way, I think that would get back at the kid without getty nasty). The teacher may not agree with this approach depending on their personality of course.

or

2. Actually be pera-pera thus making the kid look just plain silly for saying it in the first place.