Friend unknowingly brought a firecracker to school today.

The friend found it in his shoe, he thinks that it fell in while he was messing around with the firecrackers yesterday. He had no intention to use it at school and didn't even have a way to light it. He just kept it in his pocket once he found it and didn't take it out.

So if you unknowingly take something not allowed in an area like a school to one (fireworks, knives, etc) then will they just arrest you anyways without caring? What about if you go to the principal/administration and explain that you found it and had no intention to use it and all them to hold on it it?