I just now finished a reply to the thread: Mixed Messages From Ex-Boyfriend. Please read my reply to the o/p's concern. In addition, please read the articles on dating found in the Index.

Once you understand the how-to's of dating properly, I think you will have your answer. Personally, having received a telephone call in which my high school girlfriend broke up with me, I do not want to ever receive another; however, I do believe this is the best way to go about it because it adds distance and offers protection from a possible over reaction upon hearing the news. Several years later I received a similar phone call from a friend of the woman I was dating in which she gave me the heave-ho on behalf of my then girlfriend.

So, yes, do call, be pleasant and caring, and simply explain that you want to date others and wish her well as she continues to date other people.

If you don't tell her goodbye in person - it doesn't count.
Man or mouse?

I'd put this in the new relationships section first of all. A year or more can be called long-term.

Tell her it isn't working, and ask to still be friends. She needs to get used to the idea that things won't last forever, and 16 is a good age to start. Don't be an ass, just tell her that you don't think you guys are meant for each other.

All for the best