 There are skeptical premises that could yield Quine’s conclusion that the students do not know the announcement is true. If no one can know anything about the future, as alleged by David Hume’s problem of induction, then the student cannot know that the teacher’s announcement is true. (See the entry on  the problem of induction.)  But denying all knowledge of the future in order to deny the student’s knowledge of the teacher’s announcement is disproportionate and indiscriminate. Do not kill a fly with cannon — unless it is killer fly and only a cannon will work!