I think the range of answers you have got from friend, teacher and web reflect that there is not a straightforward response.
Without high voltage it would not be possible to drive the dangerous current through the body, but high voltage itself is not lethal - it depends how much current can be delivered at high voltage. Another question is how high does the voltage need to be to be dangerous?
In fact, high voltage can be beneficial to the body.... A number of medical devices use plasmas driven by high voltage. For example, here is a link to a commercial device for surgery. The plasma, driven by high voltage, can be used in surgery to remove tissue and at the same time it cauterizes to prevent excessive blood loss. It is a plasma scalpel effectively.
So the medical device is safe to use because it is high voltage, but low current (it also uses RF or pulsed DC I think, which helps reduce the current but maintain high voltage).
The real danger is high voltage and high current. But then how high is a dangerous high voltage when a large current can be delivered? Maybe over 200 V? Maybe over 120V? (don't try this at home please). The plasma medical device may use a thousand volt. So compared to this medical device dangerous voltages of 200 V or 500 V may be considered medium (or even low) voltage.
Final point, because we are thinking about human bodies we know that the resistance is high, but not very high. Pure water can have very high resitance, but the fluid in our bodies generally has some ions dissolved in it so the resistance will never be as high as something like pure water.