i haven't come across many vids on YouTube of people getting trained where i thought the trainer was helping the trainee versus just pushing his own interpretation of form onto the trainee. to me, a trainer should be able to teach safe, advantageous form but also teach the trainee how to think for him-/herself, not shut down independent thought.
i'm no trainer, and i'm not an accomplished powerlifter, but my first question would be "what form feels the most advantageous for you?" and then work from there. but you can't figure that out unless you work with moderate to heavy weights. i don't believe squatting with the bar or body weight squats are effective for teaching good form. it would almost be like saying "teaching good form on pushups is effective for teaching good form on bench press."