In the comic drawn by Schultz, no adults ever appear or speak, but the children sometimes respond to the school teacher. When they adapted the comic for animation they still wanted to keep the children-only perspective of the strip, but they still had to have the children respond to something. They could not use an adult voice or image as that would not be consistent with the environment or feel of the strip.

I don't know how they came up with the "wa wa" stuff, but I am pretty sure it was to communicate the idea that the children were interacting with an adult without creating a character for the adult.

The reason the teacher sounds like that is because someone is actually playing a trombone and muting it to a "wa-wa" sound. Hence, the "gwaku gwa gwaka gwaka gwa".

I don't know why, but if you want to know what the Peanuts kids think of it, skip this video forward to about 57 secs and enjoy!

Not for kids, but probably work safe.

Have you ever heard someone talk and they sounded like that, because you were tuning it all out? lol I have (and I wasn't on drugs, I swear!) and sometimes it feels like that when you are a kid.

I have learned to speak to my significant other by drawing pictures, since this is the only thing he ever hears me say "gwaku gwa gwaka gwaka gwa" ...... LMAO

I've read up on this, and they don't make sense because the cartoon is not about the teachers, the parents or adults it is about Charlie, Linus, Snoopy and such. They are unimportant and the speech of whop whop whop illustrates this in the movies quite well.