good news!
I wonder how Biden is taking this loss.
Probably fine because the decision doesn't impact the ability of schools to handle things like e-bullying which was the main concern in regards to the case. The decision basically says that schools have to meet a pretty strict criteria to discipline off-campus behavior, but not that it can't be done at all.
Explain further why Biden sided with the school.
Because without any further context, it looks like "Joe Biden hates students having freedom of speech".
The Biden Administration expressed concerns that a broad decision against the school could limit the ability of schools to discipline for things like e-bullying that happened off grounds. It was more concern over precedent than this specific case.
Also I doubt Biden himself thought much about the case. He's taking a somewhat hands off approach to the Department of Justice.