I'm in the school.
Means I am in the building (but I may not be a student).
I'm in school.
Means I am attending school
(I'm a student).
ell.stackexchange.com/users/2992/oerkelens
The is used to refer to someone or something that has already been mentioned, or a context that has previously been established or easily understood.
I'm in
the school This sentence is generally understood to refer to a physical location at a particular school .
"In the school" means you are inside the building, "at the school" means you are on school grounds.
"The school" makes sense if the school's identity is clearly known or obvious.
"The school" difficult to understand (or plain nonsense) if the school's identity is unknown or unclear.
I'm in school
This sentence is not specific to any school, therefore the is not used.
In this sense, "school" is understood to mean schooling , or the process of receiving an education, rather than a physical school.
To be "in school" is to be in attendance at a non-specific school.
One would specify a school this way:
I'm in school at
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Which is an informal way of saying:
I attend school at
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ell.stackexchange.com/users/11327/Substantial
We do not use the definite article "the" with the words church, bed, college, court, hospital, prison, school, university when we refer to the purpose for which they exist .
e.g. Sally went to school"
(She's a student.)
But: "Her father went to the school to see her teacher yesterday."
(He went to the school as a visitor.)
ell.stackexchange.com/users/4858/Vic
I am in the school and I am in school are different in meaning.
When I talk about being in school, it means I am a student there.
When I talk about being in the school (school building), it means I am not a student there.
Maybe I am in the school to look around the school building or to see my father who is a teacher there.
ell.stackexchange.com/users/9259/Khan
As others perfectly explained the differences between the two sentences, I'd like to state when to use which sentence.
When someone asks you, Where are you?
You could say, I'm in the school.
and if someone asks you, What do you do?
You could say, I'm in school.
ell.stackexchange.com/users/3612/chaitanya89
What is the difference between these two sentences??   I'm in the school.   I'm in school.   Do they have the same meaning or not?
Difference between "I'm in the school" and "I'm in school"
ell.stackexchange.com
<meaning><difference><usage><sentence-meaning>
ell.stackexchange.com/questions/37408
ell.stackexchange.com/users/8972/Ice Girl
I am in the school and I am in school are different in meaning.  When I talk about being in school, it means I am a student there.  When I talk about being in the school (school building), it means I am not a student there. We do not use the definite article "the" with the words  church, bed, college, court, hospital, prison, school, university  when we refer to the purpose for which they exist . Being in school means you're studying. Being in the school means you are in the building.
Being in school means you're studying. Being in the school means you are in the building.