Studying "at" implies location. You are studying at the building or campus of your school. Studying "in" school or being "in" school, is a more general concept. It guarantees that you are enrolled in school, but not necessarily that you are located in the building or campus. "Studying at" = you are at the premises of the school "Studying in" = you are enrolled and may or may not be on the school premises. That is the normal usage. Colloquially, there is some flexibility. But "at" strongly implies "place".

^^Studying [in] school / studying [at] school can both be right depending on context ^^Generally, if you're referring to "school" as the actual school building/campus/location, I'd go with [at], for example, someone asks where are you, you could answer with "I'm studying at school" ^^If you're referring to "school" as the institution (the "idea" of school rather than physical building/location), I'd use [in], for example, if one asks for your occupation, but you're still a student, you could say "I'm studying in school" ^^These are general cases based on the examples I gave. It's pretty hard to give a simple yes/no answer, just because they can both be right/wrong, depending. A few phrases below to show some ways in which they can be used: - "I'm in the school" / "I'm at the school" = both right, referring to you being in the school's location - "I'm in school" / "I'm at school" (no "the") = both right, referring to once again, the school's location; you might be confused here, because I mentioned previously that "I'm studying [in] school" can more generally mean you are "studying" as in you're currently in school as a student. This is one of the points with these prepositions being flexible and highly context dependent at the same time, because one little word can affect its use, like in our case, adding "studying" or "the." When you take "studying" out, "I'm in school" can be correct for either meaning (location/institution). Sorry if this is too confusing :/ ^^Prepositions at/in/on are pretty tricky, as differences are very subtle + flexible and changes depending on the words/contexts they're used with--you'll get a better feel for them with practice and time! ^^My examples seem very technical, but please don't stress yourself over their right/wrong-ness, because they're super similar in meaning and natives would understand what you're trying to say. I included them just to give you a sense of how they can be used. Good luck!

I think the correct is "at school" because it's position.

I was studing at school. Some prepositions is esier learn if you use it. My learn coming this way.