TLDR: Some of the other examples have suggested the term student teacher. In American English, this means something quite specific, and would be very confusing if you used it in other contexts. You could use student instructor instead.

In American English, a student teacher is somebody who is getting a degree in Education, probably in preparation for a career as a teacher, and who is teaching in a primary or secondary school in conjunction with their degree program as practice for this career. See Wikipedia.

Calling somebody who is getting an undergraduate or graduate degree in physics, and who makes educational videos and puts them on YouTube a student teacher would be very misleading.

If the student is actually teaching a class, you could call them a student instructor, since in American colleges and universities, instructor is often used for people teaching classes.

TLDR: Some of the other examples have suggested the term student teacher. In American English, this means something quite specific, and would be very confusing if you used it in other contexts. You could use student instructor instead.

In American English, a student teacher is somebody who is getting a degree in Education, probably in preparation for a career as a teacher, and who is teaching in a primary or secondary school in conjunction with their degree program as practice for this career. See Wikipedia.

Calling somebody who is getting an undergraduate or graduate degree in physics, and who teaches a class or who makes educational videos and puts them on YouTube, a student teacher would be very misleading.

If the student is actually teaching a class, you could call them a student instructor, since in American colleges and universities, instructor is often used for people teaching classes. See the Merriam-Webster dictionary.