In sentence B, it sounds like the teacher told you off because you laughed out loud, so they told you off shortly after. However, senence A sounds like the teacher was already telling you off (not for laughing but for something else) and that is when you laughed out loud, while he/she was in the middle of telling you off. So to answer you question, B got in trouble for laughing, A got in trouble for something else. Let me know if you have further questions.