I never give rides to *anyone* whom I don’t know (or who doesn’t know a passenger in my car). I know that I’ve caught flack for this attitude on these boards before, but I don’t care… safety trumps giving someone a ride.
That being said, I find this attitude a bit puzzling. The OP said:
When I was in HS our teachers told us its not tzniyus to pick up boys looking for a hitch… even if we feel bad ;).
If the teacher had said that you don’t give a man a ride for safety reasons, I could have understood it. But to say that you don’t do so for reasons of tzniyus is a bit puzzling to me.*
Once you’re willing to dismiss the safety issues, why should the mitzvah of gemilas chasadim be any less? Would her teacher also advise ignoring a poor man and not giving him tzedaka because it’s “not tzniyus” to talk to him? Would she advise not helping an old man who slipped on the ice because of tzniyus (assuming, of course, that no one was around to help)?
The Wolf
* This is assuming that there won’t be any problems with the issur of yichud such as driving in deserted areas for extended periods.