AIO if I hand in my resignation effective immediately?

I 25f have been teaching at a school for about 14 months. I teach for university entrance exams. I am qualified to teach my subject and have a degree in the subject. When I first joined, I was told I would be on probation for an year and given a raise of about 40% when I complete the year and made permanent (this is possible because they pay me less than minimum wage). I was completely fine with this arrangement.

After working here for 12 months, I spoke to my immediate supervisor and asked them if I will be made permanent. I was given vague, non-committal answers such as "we'll see" and "I will let you know". About a week later I was called in to see my supervisor's supervisor, who told me that my students have complaints about me. The complaint is apparently that I primarily target my weakest students and sometimes this leaves my strongest students idle (I do this because I don't want anyone to get left behind, as I have limited time to cover all topics. I told my grand supervisor this. I also said I'll change up my style to better accommodate everyone). I didn't think this would be something they would extend my probation over as it's a challenge that teachers commonly face. It has since been about 2 months of me waiting for an answer and I still have not received it. My grand supervisor said they would come to a class and observe to check if I have improved my teaching style but never came by. The most recent development is being told by my supervisors that there maybe a chance I get demoted to teaching lower level content (for which there is no pay raise, so I will always be paid less than minimum wage if this happens).

I have to mention that in addition to being pushed around like this with no proper explanation, I have also been given NO documentation I can use to prove that I am employed here. (eg. appointment letter).

I might be overreacting since the complaints about me might be legitimately bad enough to not make me permanent over. Also am I throwing a job away for a petty reason? I want to resign only to put them in a tough spot because graduates who are willing to work this job are rare. On the other hand I currently enjoy this job but would hate it if I'm demoted because the lower level classes have way more students in them and are probably 10 times more difficult to control.

AIO if I give my resignation verbally with no notice period? I understand my institute has the right to make any decision since I am on probation. However I can't help but feel wronged.