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Replacing the knob shouldn't be a big deal, it is usually just two screws on the inside. Privacy sets are not that expensive. Look on the edge of the door on the latch for a brand name, save up your money, and buy a privacy set yourself if your parents won't. They should though, I wouldn't want to use the bathroom with no lock. Or get the rubber door wedge for a few dollars.

you can buy the door stopper at a hardware store or walmart type. It is a rubber wedge, jamb it under the door and you would have to force the door open pretty hard to get in. being rubber it will "grab" most floor surfaces better than a home made wood block.

Go to any dollar store and buy one of those rubber stoppers that you wedge under the door.

Rubber stopper, or a metal jam to put up under the handle.

Lay your tennis shoes behind the door, or lay out a towel and stuff what you can under the door with a butter knife, this will slow then down.