Here's the sarcasm:
Speaker A:NO, i just meet you every saturday because i want to waste my time.
Speaker A comes every Saturday because he wants to learn. The sarcasm is saying the opposite of what he means. Here he is doing it to shock the other speaker and to express his frustration with the way he is being taught. Sarcasm is a form of humour and I'm not sure how is should be learned- in the UK, we use it quite a lot, for humour or to be rude. If someone says something where the opposite makes more sense, then they are probably being sarcastic.
- For Teachers