Translation of Lügendetektor

It actually happened to my cousin's youngest kid. He was repeatedly warned not to smoke in the courtyard of the school. At the end he found himself expelled from the school. It could have been an interesting experience for him though.
You can return the favor and help me too. We will acquire the new language together. I like to learn Spanish. Since high school, I have learned much more but it has been more than 20 years ago.
I will go to Barcelona on Sunday and visit a language school for Spanish for 4 weeks. I'm really looking forward to it, I'll write you more next time OK? How can I help you with your German?