That's a threat... not even remotely veiled...
Its not "I think you're probably going to hell someday" or "I think what you're doing is gross" (which are both horrible and nonsensical opinions) its "I want you dead"
It's not "I want you dead". It's "hey look at all these people who commit suicide like you, think about it, wink wink."
Which is how it's likely not a specific enough threat to withdraw free speech protections. If it were targeting particular people or encouraging others directly to kill themselves imminently, it would likely not be justified.
That doesn't mean that there isn't another avenue by which you can censure these communications, even on a public university, though. But in regards to whether they're entitled to use the public bulletin boards and what kind of forum that equates to, it's a bit murkier.
https://www.cu.edu/doc/counsel-public-forum-overviewpdf
With some brief digging, it seems like a lot of colleges do treat them as public forums they do not or will not heavily regulate, others that they will. So it's not a hard and fast rule. Penn, for example,
has a specific note:
* Think before you post. College Houses defend the right of free speech and expression on campus and actively promotes the civil exchange of ideas. In the spirit of the latter aim, students are urged to reconsider creating and distributing posters or flyers containing material or language that could be construed as malicious toward or dangerous to other responsible members of the University community.
Barnard "[does not] tolerat[e]" derogatory or discriminatory flyers; others like
UCSC have listed flyer policies but don't actually enumerate specific requirements for approval.
Relevant SCOTUS caselaw is Hazelwood v. Kuhlmeier and Healy v. James. I'd think in this circumstance they'd probably side with the school given the forum, but it's worth nothing in the former case they established lower first amendment protections in certain school fora.
Isn't this like shouting fire in a movie theater or bomb on an airplane?
Not really, and even if it were,
"fire in a theater" ain't a thing and hasn't been for decades.