probably the "student skip content structure" setting - when the structure page is skipped the pop-up is not generated on browser user-action, the teacher always sees the structure page and hits the button on the structure page to load the player - because this occurs on the browser user-action pop-up blockers ignore the action.
btw... you do know that 2.6 is no longer supported? you should spend some time upgrading your site (and implement a regular schedule to upgrade your site.)