I went to Yeshiva in Israel, and the Yeshiva told us that whatever was left behind (even things with simanim) during bein hazmanim would be hefker? On what basis can the yeshiva do this?
In חושן משפט, סימן שי״ט סעיף א׳ it says, that if someone brought something into someone else’s property without permission, or broke the conditions upon which the owner allowed him to bring it in,the owner has permission to throw it onto the streets, as long as he let the owner of the object know.
However he has no permission to cause damage to the object - חושן משפט סימן שע״ח סעיף ו׳
In the Responsa of the Birchas Reuven would seem to suggest it is assur to make such a demand. To an extent you can't even take things from genizah without trying to find the owner.
In the sefer Shareei Yosher חשן משפט סי קיד he would allow if the proper notice has been given (but only bde eved)
The above cases are discussing a Shul I fail to see a difference between a shul and a Yeshiva in this scenario