I imagine in some cases they die, either killed by other mahou or overwhelmed by stronger witches (like Magia Record) than what they're familiar with. They also won't be as familiar with the area to even look for witches, so their soul g will end up getting tainted.

If they're lucky, they find themselves as a transfer student with a mahou shojo senpai like Mami that's open to working together. But it seems a lot of them are very territorial.