The simple answer to your question is that a magnet can erase cassette tapes and floppy disks, but not compact disks. Cassette tapes and floppy disks store information using magnetic materials, and can be erased with a magnet. You may know that magnets have two "poles." These poles are sometimes called "north" and "south." A magnet will behave differently if its "north" pole points up than if its "south" pole points up. This means that we can think about a magnet as pointing in a certain direction. We might even draw an arrow on the magnet pointing from the south to the north pole. Cassette tapes and floppy disks contain a lot of small magnets. Initially all of the magnets point in the same direction, but when music is recorded on a tape or a computer file is recorded on a disk, some of the magnets are changed so that they point in the opposite direction. The pattern of alternating magnets is used to store the information, and a cassette player is basically a translator that converts the pattern into music. Likewise, a disk drive is a translator that converts the magnetic pattern into an electronic form that the computer can understand.
So, back to your question. If a strong enough magnet gets close enough to the tape or disk, it will scramble the information. This will "erase" the tape or disk. Any magnet will work if it is strong enough and close enough. Here's a though for your science project: You might test to see how close you need to get a magnet to a cassette tape before the music is erase. If you have more than one magnet, you might try this with each magnet since some will be stronger than others.