By making it a tzniyus issue, you are encouraging people to not to chesed when it is, in fact, warranted. A young girl taught to ignore a boy shivering in the rain could just as easily ignore an old man who has slipped on the ice and cannot get up.
I think there may be a misconception. Chesed is not optional. When we are approached for tzedoka, and we can give, we must. Same with other chesed. However, there are things which are lifnim m’shuras hadin. For instance, a ride for a bocher who is not handicapped. He can go home later. He can take a cab. Etc etc. In that case a girl should be taught to value tznius over lifnim mishuras hadin, since that’s what the halocha dictates.