Her school tasked her to fill a 1.5-liter PET soft drink bottle with at least 500 grams of plastic. She stuffed the bottle with tightly rolled plastic bags and small bits of hard plastic; tamped down with a stick. It was very light so I decided to re-do it, removing the upper half. I remembered from bulk solids handling that particle size could make or break an operation. Handle and store particles too big and you lose efficiency because of air voids. Reducing particle size too much and you spend too much energy (money), making your operation unprofitable. So engineers earn their pay by optimizing particle size. I used only soft plastic bags (polyethylene and cellophane.) I folded them tightly and cut them into very thin strips, then tamped them down. You tamp down the sides, not the center, in order to compact them efficiently. Nearly full after almost a day and it weighs only about 300 grams. 
PET melts at 500F, polyethylene around 225F.
Melt the polyethylene and pour it into the bottle.
Hey that’s an idea. Except that I might not be able to control temperature. Was also thinking of just pouring in epoxy glue (it’s a plastic, right?)
Another alternative is to use dense hard plastic like soda bottles cut into small pieces or strips. My thought is the thicker the plastic the better. Thin soft plastic will trap a lot of air.
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To optimize packing density you need a range of sizes. Smaller lumps can then occupy the spaces (or interstices, if you want to get all technical) between the larger ones. In a perfect arrangement of same-sized spheres the best you can do is about 74%, regardless of what that size is.
I’m an artist, not an engineer, and usually get in trouble when I tackle engineering problems. But, enh, what the heck. Maybe the OP could buy a plastic rope that weighs 18 ounces, and feed it into the bottle.
“Plastic“ as defined how? One of the densest plastics is Teflon. Not sure how you’d get your hands on it, but would make it easy if you did. Acrylic isn’t bad: maybe you have some acrylic cutlery laying around.
I suppose you don’t happen to have a few thousand airsoft rounds on hand? I think that would probably be the easiest thing to pour in.
BTW, polyethylene is around 1g/cm3, so getting 500g worth in a 1.5l container should be pretty easy.
After a little research:
Of the common plastic ropes, polyester seems to be the densest. According to the specs on this site, half-inch 12-strand polyester rope weighs 0.085 lbs. per foot. So, 15 ft. should be more than 500g.
Or maybe 35 ft. of this 3/8" stuff.
I haven’t tried to calculate how much rope could coil inside a soft-drink bottle, but intuitively it seems doable.
Stepping away from the technical issues, can the OP clarify what he meant when he wrote that he decided to redo his daughter’s school assignment? Did she ask for his help and did he just step in uninvited? And is she still participating in this project?
What is the purpose of pushing 500g of plastic into a bottle?
What is the predicted / expected outcome of this?
I’d be inclined to go with melting it into blocks, or pellets and then slicing it up
Or maybe if you can find metal ice cube trays - melt it and make plastic cubes?
How many poly pellets can you fit into a 1.5 liter bottle?
Edit: To reply to my own question, I’m seeing that 2 cups of pellets weigh about a pound (so the 2 lb bag contains 4 cups) and 4 cups is just under a liter. The full bag (900 grams) should fit in the bottle if that’s all correct.
Whoops, I needed to edit again. Forget the product reviewer who said 2 cups = 1 lb. The packaging says that the bag contains 7 cups, and 7 cups = 1.7 liters. Since the bag is just over 900 grams, you’ll be able to get enough of them in there.
Seems like a good direct solution.
If the OP is interested in a plastic source that’s probably available at thrift stores or dollar stores, he could try plastic hangers. My kitchen scale read 1.3 oz. for a flimsy hanger I had in my closet, so 14 or 15 would likely go over the 500g mark. A jigsaw or some garden loppers should easily handle the chopping.
Does this violate the board rule of helping someone with their homework?
No, it doesn’t violate the board rule of helping someone with their homework because there isn’t a rule against it.
Q: May I ask for help on my homework?
A: Yes. You may ask, but no one is obligated to do your homework for you and typically posters prefer to guide you to the proper resources so that you have the ability to do your own homework. Sometimes other posters may oblige your request.
Are you sure?
IIRC this has come up recently but, of course, I can’t find the thread. My understanding is that you are perfectly free to ask for help, and everyone is perfectly free to ignore your request. Usually the only requirement is that you attempt to find a solution yourself.
Hot melt glue would fill the bill.
So would automobile body putty, or epoxy.