How do you teach the TH sound (student is saying a Z sound) and the W sound (student is saying a V sound)?
Hey! Well, I must say I'm not a teacher But I hope this works for your students.
You know, I bet most of the teenagers love SUCKER. (stereotype
I don't like it though). So, let's get it started. First, you ask your students to hold a lollipop just in front of their mouth and then ask them to try licking it. Thus, they will do the /th/ sound right. Here's the video link:
http://www.youtube.com/watch?v=Ag4qoNzEH4w
As for the second question, I don't see why your students are having that kind of problem. ( They're obviously two different sounds). To make that sound, simply push your lips forward and blow out with a voiced sound. Link:
http://www.rachelsenglish.com/consonant_w
Well, I hope you can demonstrate how those two sounds are made to your students. By the way, I'm so glad that your students have you as their teacher. Keep up the good work, dude!