I've watched all of your vids and to be honest I haven't been very amused with the whookos episodes so far. Don't get me wrong, they're quite good and the jokes aren't bad. What I think it's lacking slightly is the climax in those jokes.
I mean so far most of the jokes - excluding a couple ones - have been spoken in monotone and it looks like they haven't been paid much attention to in the animation, either.
I think you could improve that by making some contrast into the joke. For instance: everyone else talks in a low voice and then someone suddenly shouts something funny (possibly random).
Example of such a contrast:
Hey mate, how are you?
I'm absolutely fantastic. By the way, have you ever seen -- IMMAH FIRING MAH LAZZOOORR!
Another way to create a contrast is by using different kind of colours in a scenery. Example: You have two roads, one is happy and colourful, the other a dark death-trap.
You could also draw a really weird facial expression out of the blue.
Or you could make the climax by giving a weird reaction from others. For example:
*Everyone is talking*
Someone says something stupid in a loud voice
*complete silence and staring* (possibly wind blowing a couple of leaves)
It can be anything, really. Just emphasis that "THIS" is the joke. Studying comic expressions could also be a good idea.
Also like some people already pointed out that the voices weren't so top-notch in this one. Maybe you could try adding more "emotion" to them, if you know what I mean. Don't be afraid to shout or snort or whatever!
Extremely nice backgrounds and animations. I like the simple theme. The lack of shading in characters isn't a problem. I liked the use of anonymous, black characters. The idea is fantastic. It really does have that real school atmosphere!
I also like the idea of a small group of people doing everything together.
The more successful jokes, in my opinion, were the distraction joke in whookos 3 and the zombie scene in this one.
Ps. I would write a longer review but I really should be off now.