Well, I may as well throw in from the other side.
Firstly, when I set students homework and they don't do it. Then they complain "Oh, I don't know how to do this" or "Why aren't I improving?". It's very frustrating, because my homework is designed to sort this out. I don't set massive, difficult and painful tomes of small writing under candle-light, but instead small, varied tasks designed to streamline what they have learned in class; e.g. "Watch this foreign program with Chinese subtitles for free. Write down every new word you encounter. It's an interesting program. This one is called "Friends". Next one is "Sherlock". Practise your listening. Note the culture." Every lesson I stress for them to do it, every lesson they don't.
We don't set homework because we're bored, remember, we have to sit down and mark it. Every single homework. It' very time-consuming for us, you get homework once, we get the same piece forty times. And from every class we set, not just yours.
Secondly, when students use their phone in class. They think I can't see them, it's so confusing. All I ask is that you don't use your phone for the two hours a day I have you. I know your parents pay an awful lot to the school for you to learn here, so all I want is for that to work. If you don't want to come here, change to another program.
Thirdly, when they misbehave. I don't mind talking quietly (although lads, your voice does carry), but don't piss about (and students know the line, they're not stupid). Because frankly, I have forty or so students in some lessons and I just don't have the time or attention to deal with you, so out you go and I'll focus on those who want to be here. If you don't want to come here, that's your choice, but don't ruin it for those who want to learn.
Fourthly, students who don't take part, and then complain if I try to make them take part. If you've come to a specialised language school, then you're going to have to take part in class. If I'm helping your speaking, don't just stare at me, because I can see your mouth and if you're moving.
I know there is this conception that teachers are sadists who love to embarrass you, but I honestly can't help you or give you advice on your learning if you never open your mouth in front of me. That's why I assign speaking tasks among my students, so I can write notes and say "Hey, your vocabulary is good, but you need to improve your listening, so go do this three hours a week."
Fifthly, teachers are not blind. We know when kids are bullying other kids, for example, but you know what we can do about it? Nothing. I don't have proof, I'm probably not your homeroom teacher, and while I can mention to the other teachers to keep an eye out, if you don't come to us or you dodge questions if we ask, then we're powerless in the situation. No, we don't like it, but that's the way it is.
Sixth point, teachers who should not be teachers. And, in my opinion, are not teachers. Did you become a teacher because there was nothing else to do? Or to have a power trip? Or did the phrase "job security" or "extra holidays" ever enter your mind for choosing this job? Then you're not supposed to be doing it. It takes a certain mindset to be a teacher, and to be a good teacher is not an easy job, even if they make it look easy. There are some colleagues who I look at and feel a twinge of dismay that some people have to be taught under; they've no ability to pass information, they've usually little ability in the area itself, and they turn up for the paycheck, and in this country, there is a massive problem of corruption in this profession which, frankly, worries me.
Final point, because I've gone on long enough, are some students and how they treat teachers. I know how people say "Oh, well, this teacher is horrible" or "This teacher is a *****" and so on, but I personally think that every person who says that should work in a school in the UK for a week. Teachers, and those who work in the education system, are treated like utter crap by the students and their parents. They get things thrown at them, swore at, insulted, threatened, I've had female colleagues sexually harassed, and I've had teachers break down in the common room or their office because of how they are treated. And now government attitudes have turned against teachers; "It's the teachers fault that they don't learn." No, it's because their family literally does nothing to help and we're not miracle workers; we can't fix EVERY single student who walks into our classroom.
I've heard of spitting, intimidation, and death threats, and we can do nothing about it without some thick woman coming into the office and saying "No, my child is an angel and he's never done anything wrong" and you want to grab these people and say "No, your child is not a nice person and is negatively affecting our staff and other students and this is how you've raised him!" You know how medical staff are given both respect and a lot of legal protection? Teachers really need this as well, that's why the good ones are leaving the country or profession in droves; because who wants to be paid lower to middle wage for this for any other reason than you love the job?