Lots of great answers.
Ezra, if you are both teenagers and without much dating experience, two possibilities come to mind. Either she is naive and inexperienced and doesn’t realize that sharing the costs is the right thing to do for a modern and independent young woman… or she is using you and, when asked to start sharing the cost, she’ll dump you and go looking for some other yahoo who will pay her way. It would be wise, if potentially painful, for you to make that determination early in your relationship, rather than later.
Additionally, relationships begun in high school seldom survive past graduation. In fact, most adults of my acquaintance choose not to think about their high school years, they were that bad/embarrassing/miserable. I know of only five couples who married right out of high school. One was a beanpole tall male and a petite female – they eventually divorced. One was a couple with two kids – they were hateful, backstabbing individuals, although their marriage was still going on during the period I knew them. One couple was a jerk who went back home one weekend and married his 16yo sweetheart, then threw out her birth control pills the first week in their mobile home, and got her PG immediately. He was a redneck jackass, treated her like a servant, made her wait on him and his drinking buddies hand and foot (yes, barefoot and PG) during his frequent party weekends. As soon as he obtained base housing, and she delivered a daughter, she filed for divorce and kicked him out of the house. (Aside: ugliest baby girl I’ve ever laid eyes on.) Two of them were my twin aunts. Both conceived their senior year in high school, married the fathers and dropped out of H.S. I don’t know if they ever completed H.S. but they both had four kids and are still married today.
The final couple are my niece and her H.S. boyfriend, who were married last summer. Both have jobs, and they have been living together since their graduation a few years ago. That relationship looks solid, and at least they waited and saved up some money and tested themselves by living together a few years.