Can you get kicked out of graduate school for something you published online? It depends. Depends of course on what was published. Was it racist, sexual, or threatening in nature? That's a certain consideration. Was it you're expressing in unpopular sentiment or opinion? Unpopular say political view or view about social matters? A school; they can often be overzealous in responding to or seeking misconduct charges against a student in that situation, even if they may be entirely not within their rights to do so.
A student does have certain rights, of course, freedom of speech is maybe a limited right but depending on the school's policies that may be implicated, but you can definitely face severe consequences, especially if the matter is not handled properly. An experienced attorney advisor will be your best ally to help you understand and protect your rights and to help you navigate the process. A lot is at stake for graduate students accused of such an allegation. You need to make sure that you protect your rights and interest by having someone in your corner.