Q:

In a nutshell, how can a friendship become a tragedy?

The narrator is called Karl who is a sixteen year old pupil at a boarding school. During a summer Sports Day with a rival school, he meets one of the pupils who is both a nuisance and is adorable (and is a boy his age)
Where do I go from here to use this friendship to spice up the storyline?
(There are other storylines in this story but they too aren't fully developed)

A:

I think I understand what you're after. Here are a few examples that come to mind:

Karl quietly befriends boy, boy is mocked or teased by others, boy is significantly injured or killed as a result of bullying or self-harming, and Karl carries the weight of having done nothing.
Karl befriends boy, is jealous of boy because of confidence or other issues, doesn't trust boy, and thus Karl does something that results in harm/pain for someone else. Karl is deeply guilty.

Essentially you have to bring them close, establish that they are close, and then have the reason why a split happens. Making it a tragedy means that there has to be some large emotional event, e.g. a death, something burns down, someone is significantly harmed such that it changes them, etc. This can either result by explicit action (like in #2 above) or through inaction (like in #1).
I'm guessing you're looking at using this as backstory. However, if you aren't, it sounds like you aren't planning to have this as an over-arching element of the story. Careful not to overwhelm your story with it.
I hope this helps.

