I would have to suggest that in order to minimize the confusion you seem to
be experiencing, you should get to know music theory better.
There are so many ways to explain the same thing that you need to know how
each one relates. For example, you were told by your high school teacher to
play the major scale based on the I. Another way to put is to play the
Dorian mode over the ii, the Mixolydian mode over the V, and the Ionian mode
over the I. This means essentially the same thing as what your high school
teacher said, since all of the modes come from the same major scale of the
I.
Why don't you try addressing these issues with each of your teachers? It may
make for an interesting discussion at the very least and therefore you can
resolve your confusion.