1) Zero evidence thus far has been presented that this was a case of self defense. NONE Doesn't mean some won't surface but no evidence has been provided to this point.
2) The accused was in the team tent of the victim. Why don't know why, but it's reasonably logical to conclude the accused was the aggressor not the other way around. This is supposition, based on known facts.
3) The accused brought a knife to a track meet. This is known fact. The accused took the knife he brought to a track meet and then went to the tent of the victim. It's supposition but reasonable to conclude that particularly when combined with #2 above.
4) Below is a cut and paste from a lawyer in Texas. It's a clarification around self defense and escalation.
Texas Self Defense Law - Criminal Lawyer Interpretation
So, based only on the facts we currently know for sure.
The accused brought a knife to a track meet and when to the team tent of the victim. Then, the accused stabbed the victim with a knife. The victim died.
It's logical to conclude, but it is supposition that the accused was the aggressor based on know facts.
It's logical to conclude that the victim told the accused to leave.
It's logical to conclude that the victim may have attempted to physically remove the accused from the tent. That act under Texas stand your ground law/castle doctrine may be covered.
What you can't legally do, if someone tries to remove you from a place where they belong and you don't/aren't wanted is escalate to stabbing them. The self defense doesn't play here, as the accused went to the victims "castle". Whether or not the accused intended violence prior, if he didn't leave when asked it is aggression, and the accused chose an escalation path. Even if the victim tried to push him out physically, that doesn't qualify as reason to escalate as the accused could have just left Being in the tent in the first place could easily be perceived as aggression by the victim and most jurys/judges would agree particularly if the victim asked him to leave and he didn't. That's without including the fact the accused brought a knife.
Based on current known facts, there is nothing that supports a self defense claim as bring a knife to a track meet and going to the victims tent is easily perceived as clear indication of aggression. Hell, the accused didn't even need to have the knife for his actions to have appeared aggressive.
Evidence may well surface that alters thing, but currently, based only on know facts, self defense doesn't apply.