My first thought was the type of thread the instructor was having the class use. Was it by any chance a metallic?
I know this sounds critical of your instructor, but I would urge you to check out some u-tube videos showing adjusting tension - both bobbin and top.
Your observation that loosening top tension affects how the bottom looks. Adjusting bobbin tension will affect the look of the top thread. Different combinations of fabrics and threads make it necessary to get comfortable with adjusting tension on the machine. It's a little confusing at first, but once you get the hang of it, you'll be pleased with the result.