Maybe there are good reasons not to do what I’m going to suggest, but it might accomplish what you want, depending on how you’re printing the leader’s and study guides.
Use the nonprinting color trick in this post: https://community.adobe.com/t5/indesign-discussions/fill-text-frame-with-a-non-printable-color/td-p/9049690
Create a spot color swatch of CMYK white, 0,0,0,0. Create another spot color swatch, say “Blue,” for your answer text. Alias “Blue” to the CMYK white spot color in the Ink Manager.
For the answers in the study guide, apply a character style with an underline set to regular black and the text color set to “Blue.”
With Overprint turned off, the answers should show with the “Blue” spot color. With Overprint turned on, they should disappear. The study guide should print with underlined blanks but not the “Blue” answer text. In order to print the leader’s guide with the answers, remove the “Blue” spot color’s alias to the CMYK white spot color, and print it.
That should work for printing on paper. But be aware that if you use this method to create a PDF of the study guide, the answers will actually be present, and they could be visible with some settings in some PDF viewers.