It's not clear to me when skimming the test how the body of this `it` relates at all to `class_level`.
I would probably combine both of these two tests into a single block that looks like this
```
    it "should only require class_level if teaching or TAing" do
      subject.class_level.should be_nil
      subject.should be_valid

      subject.teaching = true
      subject.should have(1).errors_on(:class_level)
    end
```

Which makes it more clear that what we're testing is
1. An RSVP can be valid while class_level is nil, BUT
2. Setting the 'teaching' boolean causes it to be invalid, **specifically** because class_level was not set.