There are thread police, just like quilt police. We learn to ignore them.
I use all kinds of thread in my Bernina, including Essential thread. If the teacher had explained that you would get more lint from Essential thread than from some others, then she would be right, but to say that the spool is too loose (when that problem can be fixed) or that the thread is fine for a Brother but not for a Bernina? Please! This would make me take all that she says with a grain of salt.