That was very wrong, in my opinion. I am strongly against art teachers doing
anything on student's work without their permission. On occasion when not
getting a technique across to a student, I will ask, " Can I show you in this
section here, and you can erase it if you don't like it?." Most of the time I
demonstrate on a separate paper, or use tracing paper on top of a student
work. I observed way too many teachers do this as a student...IT IS
WRONG!....I am sorry you experienced this.
n Malosh, Art Educator