Is it illegal to take a picture of someone sitting down?

Ok so I sent a picture of a guy who goes to my school sitting in a chair in church. It was just a side picture and I sent it to this girl as a prank. It was like 2–3 months ago. Then this girl sends a different picture of him in his car at a bakery shop. She went to the bakery shop with her parents and took the picture and sent it to me. This was like 2–3 weeks ago. Then this teacher who doesn’t like me very much found out somehow. No one has any idea how she found out. She said if I didn’t tell her the truth of what happened she would call the police on me! And Im only 13! Then the other girl got in trouble too. Apparently if we ever do anything like this again we’ll have to talk to the police. For sending a picture that doesn’t even show his face clearly! He was wearing a long sleeve jacket and pants nothing inappropriate in the picture either. Do you think the teacher who dislikes me took it too far? What we even did is it even illegal?