Are "teacher" recommendations only for teachers?

Applying to College Common and Coalition Application
1

I am applying to USC’s Viterbi School of Engineering and want to submit a letter of recommendation that my research mentor wrote me. USC has 3 spots under “teacher” recommendations, and currently I only have 2 teacher recs. However, USC says on their website that they do not want any supplemental materials sent to them. Can I use my mentor as a “teacher” even though he is not actually a teacher/professor?

Thanks!

2

I assume that you are a high school senior. A "research mentor’ for a high school student would not replace a teacher recommendation.

3

I’m not replacing it, though. USC requires 0 teacher recommendations; I am sending 2 teacher recommendations, which is the norm for most schools. I just want to know if I can use the open space for the 3rd recommendation to submit a mentor recommendation, or if that spot really is strictly for school teachers.