["One day I went to school with a stain on my pants and I didn't realize it. I didn't find it until I came home.", 'And no one told you about it?  Maybe no one noticed it at all?', "No they noticed. Everyone was laughing at me but I didn't know why."]
That is horrible!  I hope they realize that their reaction was not exactly nice.
I went to school when I was younger with a stain on my pants that I didn't know about it. People kept laughing at me and I couldn't understand why.