Can a Rosh yeshiva declare a mandatory Yeshiva wide one time fast for Bochrim for some one time purpose?
Has it been done in recent times (I mean recent like in the last 25-50 years)
IF an indiviudal can declare a personal fast, there is no question a Rosh Yeshiva (or other leader of Klal Yisrael, e.g. the “Rebbe” of a community) can declare a fast.
If he decided to do something that would otherwise not be allowed, such as to require the yeshiva to recite Hallel on his birthday, that would be different.
Of course, it’s a no-brainer.
My Rosh yeshiva did it.