How much of your friend's camping equipment can you use, eg stove, pans, BBQ, cool box? It would help you cut down if you just needed to take enough plates, cutlery for your family and could cook with your friends. You could also ask to chip in for the food so you don't have to take much yourself.
You could get changed after school then wear those clothes on Sat. Only need to take change of underwear then, and another top at the most. No need for showers, hence no towels, shampoo etc. You can have one when you get home on Sat.
Then you'd be down to tent, airbeds, bedding (put duvets on the bottom and pack everything on top, it flattens them), coats, change of underwear, crockery and wash stuff.
You could also ask the campsite if you could leave late afternoon if there's no-one expected on your pitch on Sat. It'd give you more time there on Saturday and make it more worthwhile.
I'm sure I've forgotten something vital. 