‘If I were you, I’d take your teacher’s advice with a pinch of salt.’
This means, quite simply, to doubt the truth of something, to not take it at face value.
More examples, should you need them:
– We should treat these figures with a substantial pinch of salt (The Guardian).
– Working as I do in controlled, rigorous research, the constant flow of reports I see presenting new prevention methods, is something I take with a pinch of salt (The Guardian).