Just type in Lemonaid to Google, and you will find there is a crackdown on the Lemonaid industry in America. No selling Lemonaid unless you have a licence! Anyone would think the kids were making it with a slug of contriband Rum.
Anyway, what you kids need is a vendors licence and a place to set up a home made produce market. If you get yourselves organised to do this in a co-operative I guess you only need the one licence between you.
You need to get the flip top bottles, so make sure your adults are only buying beer in Glass stopper tops. (That should stop a lot of wasteful cans). Make sure you clean them properly by hand though. We usually soak ours in Milton. (You could read 'paradise lost' while you are at it, It's ages since I read it, so to join you in a bit of solidarity I will dig out a copy, you can get it in modern English now).
Cost out all the ingredients for making your lemonaide, and add the same cost again at this stage. This will cover any production costs, and should leave a small 'margin'.
Now here is the tricky bit. You have to get your bottles back, because they will go in short supply. It doesn't matter if you get new bottles being re-cycled in your direction from the beer drinkers. What you need are enough bottles to get a full 'operating plant' going. Don't over produce, it's better to sell out than have product wastage. Besides, it also creates demand, and you may find customers starting to place orders.
I would suggest a special offer period of 'bring us 4 clean bottles for a bottle of free lemonaid'. However only set aside 10% of your 'stock' to trade for bottles, as you need cashflow to keep making lemonaid.
You need to establish a culture of returned bottles. When we were kids, if you took your empty bottle back you got money off the next bottle. You need to add enough money to the price of your bottled product that makes it a good idea to bring the bottle back to you. Say it works out at $1 for the product, add 50cents for the bottle. If the bottle isn't brought back (the customer breaks it!) you make an extra 50c. If it is brought back intact and clean, you simply sell your lemonaid for the $1 you expect to get for it.
Don't buy the bottles for cash! Only swap them for full lemonaid ones at the $1 price as a regular transaction, or 4 clean bottles for a free bottle of Lemonaid if you need bottles. Only Trade up to 10% of your Lemonaid like this though.
Don't spend any money on anything but making the lemonaid, and increasing production. Make sure it is very good lemonaid. Try making other products from oranges, or ginger. So you are widening the appeal of your product. You will know if you like it! Don't drink all the profits!
Keep strict accounts of all transactions. Remember you need to sell about 80% of your Lemonaid for cash, but at the same time be prepared to barter for other items with the other 10%.
Now I assume that if you are 'trading' with money, there will be all sorts of tax and fiscal problems to deal with. So this is where you teach your customers to barter. What do you actually want or need? What is there a market for on e-bay for instance? Books, CDs, Toys, do some research and see how you can convert your very legitimate lemonaide into another commodity that is not perishable. The more quickly you can just swap lemonaide for something that doesn't have a shelf life, the easier it is to trade in other items of value.
Then get the guys who like selling stuff on e-bay to sell the items you think you can trade for a profit, and get your cash back for personal use. I think in essence this is money laundering! You better check with the economics department as school to see if you can do this.
However as it seems this is mostly what happens on stock markets, It had not better be one rule for grown -ups that work in markets, and another one for people who don't!
And no skyving off school to do this either! You might want to put in some serious thinking about this, and see if you can make it work, It beats just hanging about doing nothing, make sure you get your homework done as well, you need the maths to run the accounts.
Why not try growing some windowsill herbs while you are at it, Basil, Chives, and Coriander are always useful in the kitchen. These just cost a packet of seeds, some growing all purpose compost and re-use some of your plastic tubs like yoghurt pots. You can always paint them, and write on the outside what they are so you don't get your seeds mixed up. These grow all year round indoors.