I used to deal with difficult students that way: tell 'em they have a last chance, that their opportunity is real and you hope they take it, but that it really is the last chance, and that whether it works is up to them. If they believe you about the "last chance" part, it can work, but if you have given them a succession of "last chances," you don't have what it takes to teach a roomfull of adolescents. When I used to mentor new teachers, I emphasized three things regarding "discipline": 1) Never forget: they have you outnumbered; you will never outshout them, 2) classroom discipline is 100% the result of the sort of relationship you build with the students, 3) Be in charge, but don't make such a big show of your authority (almost always as a result of insecurity, which they will see immediately) that you piss them off (see 1).