Sheila is essentially using the boy when she agrees to go out with him. She is a self-absorbed young lady, as is evidenced by the way she talks only about herself on the canoe ride to the dance, and, as she tells the boy straight out,
"I'm playing outfield...I don't like the responsibility of having a base."
Although she is talking about baseball, her words make the game a metaphor for the way she lives her life. Sheila Mant is out to have a good time, and it really doesn't matter to her whom she is with, as long as she gets what she wants at any given moment. When the boy asks Sheila if she wants to go with him to the dance, she really doesn't "seem to see (him) at all." Her only response is to ask if he has a car, and when she asks him this question, she still does not even look up at him.
Sheila's real motivation for wanting to go to the concert becomes evident when she nonchalantly tells the boy that "Eric Caswell's going to be there" as he paddles the canoe towards their destination. Sheila is interested in Eric, and she is basically using the boy as a means to an end - to get her to the dance so that she can see this other guy. Totally oblivious to the sacrifice the boy has made for her, she achieves her objective, and gets Eric Caswell to take her home. It matters little to Sheila how the boy feels; other than the short moment when she finally actually looks at him and tells him he's "a funny kid," she gives him little consideration, so absorbed is she in her own agenda.