Good question. The issue here isn't so much about what's grammatically correct as it is about what sounds natural. I'm American (native English speaker), so what I'm going to say may not hold if you're British or Australian. That said, in American English, it's very common to just leave the "the" out in this sentence; in fact, the sentence sounds more natural that way. However, it isn't technically wrong to say "the school." Interestingly, kids who are native speakers often say the "the" (it sounds kind of cute--as if they need to specify what school they're attending!). Additionally, it would be even better in this sentence to use the word "at" instead of "in." The sentence should be, "All the teachers at school are mean to me." I would also add in "of": "All of the teachers at school are mean to me." To my ears, this is the best option. Hope this helps!

No, you do not need to use "the", because you are not talking about one specific school, in comparison to other schools. In this context, the child is talking about his experience of 'school life', not about the physical place, the buildings, this particular institution etc. I (from the UK...) would not say to my daughter, "Did you have a good day at THE school?" However, I might say to a friend, "Does your son go to THE school opposite the church?" - because I'm talking about a particular school, in a particular placeand not any of the others. Of course, you can use "the" in this example if you wish. It's not "wrong". It's just not necessary. I would say that it's superfluous. Actually, I agree with Tula: I think most people would be much more likely to say "All the teachers at school..."

I couldn´t resist answering this question... I´m a native English speaker (and a professional teacher)... To be honest, you shouldn´t get too hung up on the "the" in your example.... it doesn´t really matter whether you use it or not. It´s correct whether you use it or to leave it out. Remember the context: You are talking to your son. Therefore a lot of background information can be assumed between the two of you. You know what school he is referring to perfectly well, so the comment about it being a "specific" school is a little bit redundant in this case. It´s a specific school (his school) whether you use "the" or not. With regard to the previous commentor... I do acknowledge that the word "the" is usually used for specific things and things that have already been referred to in a conversation. I´ve heard native speakers use the "the" and not use the "the" in your example.

Thank you. Would you say 'in the school' or 'at the school' here?

I agree with Charlie and Tula. A more natural sentence would be either "all the teachers at school are mean to me" (as they've suggested) or simply "all the teachers are mean to me." To me, personally, "all the teachers in school" sounds odd, because the phrase "in school" usually means "attending school." At least in the US, it's not normally used for describing teachers. So if I had to use the preposition "in," I would probably say "in the school" rather than "in school." But "at school" sounds better than either of those options.