Why didn't Dumbeldore accuse Tom Riddle of being behind the chamber opening? Although he already knew that Riddle can speak Parseltongue (from their first meeting at the foster house), he could at least doubt Riddle and help Hagrid to prove his innocence.
There was no evidence showing any wrongdoing by Tom Riddle. Accusing him would have been pointless. Dumbledore himself alluded to this in Chapter Seventeen of Half-Blood Prince (my emphasis).
"Rigidly controlled by Riddle, they were never detected in open wrongdoing, although their seven years at Hogwarts were marked by a number of nasty incidents to which they were never satisfactorily linked, the most serious of which was, of course, the opening of the Chamber of Secrets, which resulted in the death of a girl. As you know, Hagrid was wrongly accused of that crime.
This does imply, though, that Dumbledore may have had suspicions at the time (unless he was only able to see the broad picture with the benefit of many years of hindsight); however, accusing someone merely on the basis of suspicions would have been improper. Perhaps especially with what we know of the sorry state of the wizarding justice system (think Hagrid, Sirius, the memories in Goblet of Fire, Hokey, Morfin, etc.) Dumbledore would not have wanted to take the risk of having someone convicted with no real evidence.
Additionally, starting up with Tom Riddle with no evidentiary backing might not turn out well even for Dumbledore. The rest of the staff adored Riddle. He was the handsome orphaned top student, and Dumbledore making baseless accusations against him may have come off looking bad.
Instead, Dumbledore did what he could to help Hagrid, and filed away the information about Tom Riddle for later use.