But DD will be 9, and I'm a b-day party nut.
We're going to divide the house into different "rooms".
First they are going to have science class. We will so some simple experiments. Will it float? and the getting an egg into a jar one. I'm thinking about giving groups of girls materials to do an "egg drop" experiment off the 2nd floor balcony.. but like I said.. I'm nuts, and these girls are quite a bit older than yours.
Then we will go to b-ball practice (we have an indoor LT hoop.. you could use anything..) and we'll see who can shoot a basket with their back to the hoop, and play a "hot potato" type game with a bball. (if the music stops when they are holding the ball they will have to do something silly.. "walk like Sharpay" for example.
Next to "detention" where we are making no-sew pillows out of HSM fleece.
At "Lava Springs" we will do a relay race with lunch trays and play food
At "Scholastic Decathalon" they will be divided into 2 teams and we will compete with HSM trivia questions
We will have cake (easy.. she just wants red with "High School Musical" written in yellow letters.. much easier then when she wanted "Steve" drawn freehand on a cake for her Blues Clues party!) We will eat off styrofoam trays and have small containers of milk like at the cafeteria.
Last is the "auditions". HSM karaoke.
I have HSM calendars I got on clearance, and popcorn bags from Dollar Tree that I am putting microwave popcorn, basketball containers filled with red/white M&Ms, baseball pencils, and HSM scrunchies, and HSM magnets that we are making. We're making a banner that says "Go Wildcats!" to hang by the front door, and doing all the streamers/balloons red and white. We asked the guests to dress in red/white too.
For the invite we made up a sheet that said their homeroom teacher was Mrs. Darbus, and we included their "class schedule". The thank you note will be their "report card". A lot of this was my DDs idea. We love this kinda stuff..
You can always freeze dance to HSM music.. I read somewhere about "hide the cellphone" if you can find small candy phones (I didn't have any luck..)
Have fun.. I'd love to help more if I can.

I think oriental trading has decorate a megaphone project kit available, and cute inflatable microphones too.