That's basically it. Her corruption needs to be 31 or higher for the option to appear.
It's a little tricky because you need to do some things in a particular order at the beginning (before the first exam), but then just push her towards corruption and you'll get it.
The test difficulty must be high, so Soushi fails the exam, and then, after Ayumi fails as well, don't increase her grades after. Aside from that, just choose corruption oriented choices and you should get her corruption high enough.
