What is the definition of a protest? The protesters were yelling for their support for illegal aliens. He was yelling back his support for legally deportation (protesting her opinion). The student, like many high school students, don't like to be challenged on their opinions so she retaliated by throwing a water bottle. Again what he did by chasing her was stupid and unnecessary. But once again it shows you that you don't know what someone else is capable of when you confront them.
You do understand that expelling the student doesn't have to be for the year. It can be as short time. A better term should have been a long term suspension. Regardless, there are options.
There are easier ways to stop the protest than using armed guards. Instead of telling parents that there would just be a slap on the wrist, let them know that if a student leaves campus they will be subject to a multi-day suspension. So instead of the parent saying its ok to miss the afternoon, the student will miss 3 days, they would be more inclined to tell their kids to just stay in class.
Clearly suspensions work better in grades k-7/8 because the parents are inconvenienced. But suspension is still an effective tool for undesired school behavior.