What kind of decorated cake should I make for a school function?!

One time I made a cake that look like a beehive. That would be springy. I took a dome shaped cake and covered it in vanilla buttercream. Then I took yellow sugarpaste and made it into a rope and twirled it around the cake. Then I made bees by rolling one black and yellow balls of sugarpaste. I took two black and a yellow and squished them together and then I squished a smaller black ball of sugar paste on one side and then I drew a face on the smaller black ball and then stuck them around the cake with floral wire. Oh I also made a ladybug cake once with sugarpaste icing.

For a Cake Walk there are several good selections you can do. A Hello Kitty Cake is good choice like you suggested.
But here are a few of mine suggestions you mite like to think about.
Cup Cakes made in form of a shape. There easy to eat, and fun to decorate.
If you know how to decorate. a Doll Cake is another good choice. Make a Bundt Cake [ usually a yellow cake ] put a Doll with in the middle of cake [ Barbie or Tall thin doll that fit in the middle of the Bundt Cake, and decorate according with butter cream icing.
Spiderman is also another choice for the boys. You only need blue, red, black and maybe green coloring. For the boys is usually a chocalate cake.
A Marble Cake is another good choice.
Something for St. Patricks's Day.

Cakes:- Upside Down Apricot
Topping
100 g unsalted butter
12 Apricots halved and stoned
1 Juice and zest Lemon
1 cup caster sugar 1 teaspoon vanilla essence
Cake
100 g unsalted butter
1 cup caster sugar
4 Eggs separated
1 teaspoon vanilla essence
1? cups Plain Flour
2 teaspoons Baking powder
Topping
To make the topping, place the butter, sugar and lemon juice in a non-stick frying pan, with a metal handle over medium heat and melt.
Cook gently for 2 minutes, stirring occasionally. Increase heat and boil remaining liquid in the pan for 5 more minutes until a rich caramel forms. Remove from the heat and arrange apricots face down in the pan.
Cake
To make the cake, preheat the oven to 180C.
Place butter and sugar in a bowl and cream until light and fluffy.
Add egg yolks one at a time, beating after each addition. Add vanilla.
Gently fold in the sifted flour and baking powder.
In a small, clean, dry bowl; beat egg whites until stiff. Fold through cake mixture with a large metal spoon. Spoon evenly over the apricots and smooth with a spatula.
Bake cake for 45 minutes, or until a skewer inserted into the centre of the cake - comes out clean.
Remove cake from the oven and leave in the pan for 5 minutes. Turn cake over onto serving plate and serve.

Here ya go..
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Heh..
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Make an alligator or dinosaur. Those stay cool.

I made a cake once that looked like a big hamburger.