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](https://en.wikipedia.org/wiki/Student_teacher). 

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.