You are talking about a long time for a relationship to last, because it is a time of life when you're going through so many changes. Statistically, I wouldn't expect you to be together long enough to marry. Having said that, it is hardly unheard of for a relationship to last so long. I have many friends who were sweet hearts in high school and are sweet hearts still, 30 years later. It's the way to go, if you get that lucky! :thumbsup:
I think it is enough that you both know that your intention is marriage, when the opportunity presents itself. I don't think the promise ring is necessary or even a good idea. There also needs to be a recognition that this is a time of life when you have to be ready for the possibility that one of you could change your minds. If you have exchanged jewelry and wear it, if you have an understanding that you will talk it over before one of you flirts or engages in the "rituals of courtship" with anyone else, then that is enough. It is not wise to be too locked in just yet. Let that wait for your engagement (which IMHO ought not be longer than a year and a half, in the absence of a military deployment or something like that).
If you have not done it already, this is a good time to work out what your boundaries are with regards to friendships with members of the opposite sex. Some couples admit none, save that they socialize with other couples. My husband and I admit friends of the opposite sex, but only of the type that an unmarried man or woman might have with a priest or religious: That is, they meet in public, never secretly, they avoid anything that could give the impression of courtship, and others are aware of the meetings and always invited. In our case, this means that my husband is always aware and invited if I meet a male friend for lunch, and I don't socialize with any male friends with a frequency or under circumstances that would be scandalous if it were with a priest. For instance, you might have coffee every day before or after class, with a classmate. In the absence of a standing event that you both attend, though, you wouldn't arrange your schedule so as to meet her for coffee every day. That is a bit much for "just a friend".
These are things that each couple works out, but a good thing to work out before you head off to college, or before she heads out into the workforce and leaves you in college. I think that it is good to have friends of both genders, but friendships in which you might develop an attraction that you could not ethically pursue have to be treated with prudence, or else disaster is likely. Attractive people have a way of attracting people. It is just the way humans tick.
Having been in high school relationship that did not work out, though, I would give you this advice: You need to be completely honest about whether you are feeling you should date other people. Promise each other that if you have this desire in a persistent manner for longer than a month, that you will admit it, and give each other permission to date others. I would suggest that you wait, though, and don't actually go on a date for two weeks or so after you get that permission. Spend a little time without going on a date before you date anyone else. It may be that you only needed a little breathing room to be yourself, and not a different relationship. Whatever you do, though, do not get emotionally entangled with a third party without having had this discussion. Only fools and the cruel let themselves be in more than one relationship at a time! Don't let that be one of you!