Is a Yeshiva allowed to ask bucherim for knas for coming late in the morning? If yes, what’s the limit? Is the Yeshiva supposed to give back the money eventually? If the Yeshiva does not give back the money, are the bucherim allowed to take money from what they collect in Purim?
Yes, a yeshiva is allowed to take a knas from bochurim that come late in the morning etc. It is like takonas hakahal, that the elders of a community can decide on certain rules regarding the community, and to punish if someone doesn’t obey the takana they will have to pay a knas, and the knas is binding. Besides, when a bochur is accepted to the Yeshiva it is as if he accepted on himself the rules of the yeshiva, including the punishments and knasos. The limit is what the hanhala of the yeshiva decides, and the yeshiva does not have to give back the money. Obviously, the bochur is not allowed to take from the tzedakah money collected for the yeshiva, because that is simply stealing.
Tashbetz 2-5, Teshuvas Ra’em 57, also see Teshuvas Rivosh 518 where he says regarding people that were not coming to davening on time, and he says that they should be given a knas!.