Looking on the internet deeply has found these results:

i'm in the school is the most popular phrase on the web.

i'm at the school 18,730 results on the web	i'm in the school 13,250,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Now I'm at the School of Information (SI) to study library and information services, and aiming to work at the Library of Congress Congressional Research�... Dec 18, 2009 ... The real difference is I'm at school versus I'm at the school, or I'm at church versus I'm at the church. Without the article, you're there to attend,�... Jul 6, 2014 ... I've got two primary goals for my personal brand: (a) build a fantastic network while I'm at the school, and (b) to keep writing and use my blog as�... "I'm at the school" (the known school, but not one where I have an official role) "I' m in school" (instead of being in the workforce, I'm a student)	I'm the fastest in my school. I'm in diving school across the river. I'm in school during the day. They say I'm the ugliest in the whole school. I'm in charge of the school raffle, and we really need new band uniforms. I'm in the school looking for you, why weren't you at my place? Sorry, ma... it's just that I'm writing an article in the school paper and I'm looking for ideas. When I'm in the yard at school.

i'm at the school

18,730 results on the web

i'm in the school

13,250,000 results on the web

More popular!

Now I'm at the School of Information (SI) to study library and information services, and aiming to work at the Library of Congress Congressional Research�...
Dec 18, 2009 ... The real difference is I'm at school versus I'm at the school, or I'm at church versus I'm at the church. Without the article, you're there to attend,�...
Jul 6, 2014 ... I've got two primary goals for my personal brand: (a) build a fantastic network while I'm at the school, and (b) to keep writing and use my blog as�...
"I'm at the school" (the known school, but not one where I have an official role) "I' m in school" (instead of being in the workforce, I'm a student)

I'm the fastest in my school.
I'm in diving school across the river.
I'm in school during the day.
They say I'm the ugliest in the whole school.
I'm in charge of the school raffle, and we really need new band uniforms.
I'm in the school looking for you, why weren't you at my place?
Sorry, ma... it's just that I'm writing an article in the school paper and I'm looking for ideas.
When I'm in the yard at school.