1- Generally, I'd say 'the wrong person' because there's a specific reason why they are wrong. However, it might be possibleto thinkof a way of using the indefinite article- I can't come up with anything, but might after I've had my coffee.
2- He took a wrong turning and got lost opn the way to my house. (Many wrong turnings)
3- I think you could use either here.
- For Teachers