If it was head trauma, that doesn't always kick in right away. One example I can think of right off the top of my head was Natasha Richardson. Hit her head in a skiing accident, walked away seemingly fine and refusing medical assistance. Complained of a severe headache, was flown to a hospital about two hours later. Died two days after the incident.
True, then again Nex's own words were the people who did it were Freshman, Nex is a Sophmore, Nex thought they were insulting them so threw water over them, the 3 girls then attacked Nex because Nex threw water over them (before we get into the "it's just water" yeh it is but also such actions are considered a minor assault thus actually beyond the grounds of provoked assault) and Nex admitted throwing one of the girls, it's implied head first, into a towel dispenser. By the sounds of Nex's recollection of things on of the girls then sweap Nex legs and they went over hitting their head potentially.
What we also know, Nex did got to the hospital and receive medical treatment.
So to lay out the less comfortable aspects of this case.
Nex believed the girls were mocking them and had been insulting towards them somehow already, it's not entirely laid out how or what they'd done before this (hate to say it but lower years mocking higher years in the school, not uncommon anyway).
Nex then in a bathroom believing they overheard the group mocking them went over and threw water on them.
They them retaliated against Nex.
Nex then grabbed on of the girls and threw them, implied to be head first into a towel dispenser.
Nex's friends jumped in
One of the girls seemingly managed to sweep Nex's legs and that's as much as we have of the fight.