#1:Main character realizes that the intended love interest has been there all along and been there for him/her and is better than anyone else he/she could have asked for.
#2: Sometimes in books, after all the crazy stuff in the rising action and climax has happened, the characters just realize they're right for each other and that they'd be a good couple (maybe just have them start to hold hands at the end--nothing big)
#3 Main character realizes he/she loves the intended love interest because the intended love interest starts liking someone else or is hurt (don't really know what kind of a book this is)
You don't have to put romance in there to make a good book though, and maybe "easy romance" might not be the best idea; that way you can avoid using a cliche and make your book more unique:)
I would think if the characters are 15 they would do what 15 year olds do. Walk to class together, hang out with friends, sit together at football games. I'm old so I'm not sure if kids even do that stuff anymore-lol.