I'll start sating that I do not have much experience myself, so i'll be using knowledge I got from friends and/or reddit, and also A Lot of personal opinion. First: you are overthinking this. There is no secure way of starting romantic relationships, you will always be rejected at least once even with the perfect pick up lines, build up or with years and years of friendship behind you and your Interest, 'cuz it not only depends on you but also the other person you want to take out, if she is interested too, how she values your company and/or if that person wants to be in a relationship on the first place. What im trying to say here: Dont make rejections a big deal, less at your age, being rejected is normal and shouldnt affect you more than being sad for a day or two, girls come and go, more at high school, just be yourself and someone will someday come. Second: Pick up lines vs building it up. I think it depends more on what you are looking for, at your age, I think pick-up lines and flirting are more of a go-to. Build up is the perfect method if you are seeking more long-term relationships, something more "mature" as someone else said but that doesnt mean you cant use it or that you are wrong for prefering this one! Usually getting to know the person helps you clear your feelings, what you really want from her and if there is any compatibility. I personally like this one more than using pick-ups, as it lets you be honest with yourself about what you truly want. OP if you prefer this method then go ahead, be honest with yourself more than anyone else, dont force a change if you dont want it just cuz the other method worked for someone else!

Now, pick-ups and flirting are more for a short-term thing, perfect for young people that are discovering their sexuality and their relationships dont usually last too long, one night stands and the like, as you dont know really well that person and you 2 may not be compatible, but still have interest on either her looks, a common hobby or whatever that attracted your attention. They are a great way to get a girl on clubs, for example. Of course, you may hit a girl that has honest interest in you and start a serious relationship, or you can use them to declare your romantic interest. OP, if you dont like this method please dont force yourself to use it.