Since many answers aren't actually answering the question...

My comment is largely wrong!! See the reply below

~~Most places require a permit to assemble en masse. There are "free speech zones" where you can create large gatherings without any kind of advance notice or permit or whatever. Most universities have a free speech zone towards the middle of their campuses. Cities will also often have at least one but somewhere that doesn't inconvenience commerce, like a park or near city hall.~~

~~Most mass assembly requires a permit and sometimes a fee, even in public places. Following this prevents arrest by "disturbing the peace" or other such laws, usually.~~

~~How this squares with the first amendment is interpretation. Individual freedom of speech is protected except very specific public order and safety things, e.g. calling for violence. Coordinated, mass freedom of speech is perceived as a fast path to rioting.~~

~~I'm not saying this is right, but this is my understanding of how things work. I'm not a lawyer or an expert in these matters. This is just what I learned from activist friends in my university time ages ago.~~

As for expulsion, public universities are run by states. To my understand, Trump has no legal mechanism to do this. He's just talking out of his ass or expects to bully public institutions into expelling students by threatening to withhold department of education funding... but he's planning to kill that anyway, so 🤷‍♂️