I think it would depend on how a jury viewed the "challenge" to her audience.
The general rule for self-defense in Texas is that the person needs to reasonably believe that force is immediately necessary to protect herself from someone else's use of force. I think a jury would find it reasonable to believe that someone forcefully attempting to steal your gun was planning to use it against you.
More importantly, the law generally presumes that that belief is reasonable if the person is being robbed, assuming that she isn't otherwise engaged in criminal activity. Since openly carrying an AR-15 is -- as far as I know -- legal in Texas, I think she'd probably be fine.
But: The law also says that the use of force is not justified when a person consents to the other person's use of force, or if the person has provoked the other person. So now you have the question of whether the student's challenge constitutes a provocation or consent to the use of force.
I think you can make a decent argument for provocation, which means that "the defendant did some act or used some words intended to and calculated to bring on the difficulty in order to have a pretext for inflicting injury." Neal v. State, No. 12-14-00158-CR, 2016 WL 1446138, at *11 (Tex. App. Apr. 13, 2016).
You might also make out a decent argument for consent, which doesn't necessarily seem to require that the parties exactly spell out the rules of engagement, just that there is some kind of agreement between the two parties.
In one case, for instance, a defendant tried to argue that a fight had gone beyond the rules because one party used a chokehold and knocked the other out. But the court said that the only actual rule agreed to was that there would be no weapons used. Padilla v. State, No. 03-07-00513-CR, 2008 WL 5423139, at *2 (Tex. App. Dec. 31, 2008).
That makes me think that as long as there's consent to some kind of fight, you don't necessarily need rules, though you do need to abide by them if you agree to them. So what's the scope of consent in this case? If we say that she's agreed to the use of force by challenging people to take something from her, and she hasn't said how you can do it, can you do it by any means you choose? I don't think a court would let someone shoot her to get it, but maybe they would be allowed to pry it out of her hands.
So all of that is a long way of saying that this is a tricky question, and that any decision would probably depend a lot on the specific facts of who she was talking to, what exactly she was saying, how she was carrying the gun, and so on.