In school, at school

Yes, there's a difference, but not much of a difference.

When we think of someone who is enrolled as a student, we use "in school".
My five-year old will be in school for the first time this fall.
Did you see that little kid running around in the neighbor's yard? Believe it or not, he's already in school.
Summer is over. Now all the children are in school again.
Paul! I hear you took a job with IBM after graduating from high school. -- No, actually I'm still in school. I'm attending the University of Wyoming. I'm just in California now for a week's vacation, so I'm not at school now.

We we think of someone who is physically present on the grounds of the school, whether outside or inside of the school, participating in the activities of the school, we use "at school".
I wonder what Johnny is doing at school today.
What kinds of things do you do at school?
Do you play football at school?
Where is your little daughter? -- She's at school. -- On Saturday? --Yes; they're at school rehearsing a play she's in.

When we think of someone physically inside the school building, we use "in school" or "at school".
Most students are in school at this hour of the day.
Most students are at school at this hour of the day.

(These illustrate usage in American English.)

I can't decide. They both sound fine.

To me, the very slight difference is this:

1) talks about students who attend the school and participate in the activities of the school.
2) talks about students who are enrolled in courses at the school.

I find it impossible to decide which I would say myself in a situation which required me to say one or the other! I'd probably say the first, but who knows? They are extremely similar to me.

Remember that these are personal impressions. Other speakers' impressions could well be different.

Also recall that there are many cases in English where two (sometimes more) prepositions are equally acceptable without any significant difference in meaning. The choice can be as arbitrary as which town you live in! If everyone in your town uses a certain construction, you will find yourself using it too.

You're welcome.

Note that you (and Taka and Hela and quite a few others) pose questions that I (and others, I think) find very difficult. They seem easy, but appearances are deceiving.

Let me explain with this: A millipede was once asked to explain how he could possibly manage to walk. "With all those legs, it must be difficult to coordinate them all. How can you do it?" As a result of trying to explain and demonstrate it, the poor little millipede got himself all twisted into a ball. He was so sore, he couldn't walk for a week!

The moral of the story is that it is usually easier for us to make word choices in our native language than it is to explain to a learner how to do it.

It's also why the first rule of a comparative linguist is, "Never trust a native informant"!

CJ

We we think of someone who is physically present on the grounds of the school, whether outside or inside of the school, participating in the activities of the school, we use "at school".

The above from Califjim.

My brother is a teacher.
Let us say now he has gone to the school because he has to conduct some lessons.

Where is he?

He is in school.
What I want to say is, if he or she is in school participating in the activities of the school, it would be correct to say he/she is in school.

By the same token, if someone is in prison and spending time as a prisoner, we say:

He is in prison.

If you are a member of senate and have gone to the official building for meeting or for other activities, you could say:

He is in Senate.

Califjim uses the the word 'at' instead of 'in' these situations. It might be AmE.