Well you are in a mess huhhh.
Obviously I can't see what's going on.. However
This worries me into assuming that it may be your teaching style - perhaps you aren't best suited to this age group and if so then move to a more suitable school?I haven't had classes this bad since I taught in Japan. Any ideas? I've been teaching in China for five years now and though I've had problems, I've never had students as unmotivated as these.
When you say "remedial questions" and "they don't understand what to do .... for easy tasks" this makes me wonder whether their level of English is higher than your lesson aims..
But without seeing what's going on it's difficult to suggest ideas, like throwing or smashing an old hand phone deliberately placed with a good student in "no man's land" but that might mean you get beaten up.
You could threaten to mark the unruly mob as absent even though they are there unless they get their shit together and switch off their hand phones.
You could ask a senior (one who puts the shits up students) tutor to sneak in at the back and observe.. But that might screw you up if it's your teaching style that's a fault.
You could ask a senior teacher to speak with each one individually to find what's causing all this.. I doubt if they are the same in other lessons.
Of course being the sole foriegn teacher that's supposed to be where you can shine because you aren't being compared to other foriegner teachers.
So I'm thinking that you are teaching the wrong age group..