Hi! Both "in a different school" and "in different schools" mean the same thing but when you want to start your sentence with "we/they", you need to used "in different schools" in order to show that the people you're talking about study in different schools. For example when you say: "they are twins but they study in different schools", what you mean to say is that they go to different schools and don't study in the same school. But if you want to say the same thing but start your sentence with a singular pronoun (he/she/), you need to use "in a different school". Example: "Mike is my age but he studies in a different school." but look at the same sentence now but this time with a plural pronoun: Mike is my age but WE study in different schools. If you say: "we study in a different school", this means that there is a third person besides you and Mike that studies in a different school from both you and Mike together. Example: Mike is my age and we study in the same school. Jerry is also our friend but we're in a different school. (Mike and I are in a different school from Jerry)

Thank u so much, that's very nice of u.