1. Quiet: This is one of the quietest gay theme I've seen so far. Two lead actors, Carlos Echevarría and Javier De Pietro pretty well fit it. But, their feeling and character delivered well by good acting, cinematography, and of course music.
2. Love: Interesting that director and also writer (Marco Berger) share point of view about love between adult and teenager, son and father, underling and boss, employee and director, student and teacher. There is always a gap between that two degrees. Marco Berger choose to combine that two degrees by growing love between student and his sport teacher. I guess everyone ever had some crush to their teacher but the tricky point, not everyone, just a few who dared taking chances step the line and take actions to get to know their teacher in very close way. The extreme part, to make love with their teacher. There are differences between crush and love. But, what I see in this film more than just crush. I guess Martín already know his teacher Sebastián far from just become his sport teacher. That's why he was taking chances to get to know his teacher closely by lying.
3. The Truth: Being lying there are always consequences. The fact that Sebastián was straight and he sense something "wrong" with Martín, he made some wall to keep him stay away from Martín. The truth end up tragic. Sebastián loose control and beat Martín because he cannot accept that facts that Martín lying to him and abuse his trust and kindness. Martín fall apart but hold himself to keep it up and prevent Sebastián being fired from school.
4. Forgiveness: I think there is no connection between Martín accident to his "break up" with Sebastián. Just a big coincidence. A very big coincidence. Sometimes, accident happen, but death is always the worst part. Sebastián turn to fall apart. Martín absence was killing him form inside. He take some of it as his fault. He break school's swimming pool window and drinking. His mind trying to cope that situation by imaginary of Martín still alive. He was sorry and Martín just smile. It was enough for him.
Addition, I don't think that event turn Sebastián become gay. After all, he is trying not be ignorance man anymore.