I helped hire RA's in college, and to me this seems similar (a position of leadership/responsibility among peers). From that experience, I learned to ask for specific examples and stories. We didn't always know them, so this prevented them from giving us generic answers they thought we wanted to hear and we had better insight into the actual person. For example, instead of #3, maybe try: "Describe a conflict you had with another student in the past and how you resolved it."
Here are some others along the same lines that we used during interviews:
Tell me about a time when you used leadership abilities to make a group project successful.
Why do you want to be an STO? (to make sure they are applying for the right reasons)
What do you think is the most difficult aspect of being an STO? (to make sure they understand that leadership roles mean additional responsibility and won't always be fun).
What are some specific goals you'd like to accomplish as an STO?