Are you doing these for random schools or the school you work at? If its the school you work at part time then its highly suspect....seems like shit these English school would try to pull.

And why not charge for demo classes? A least do them at half price or something. You should always get something for your time.

Yep! it's a scam. They want something for free. You've spent a lot of money getting your authentic docs authenticated. You flew about 10,000 km. You probably paid for a medical and numerous other things up front. Now your going to work for free???

Always charge for demos. If they won't pay, they're not worth your time anyway.
Your time is money.
Don't let them take advantage...because oh trust me "they" will.

Demo classes (real demo classes) aren't a scam. 

First, be a real teacher. Real teachers dictate terms and can be effective when it comes to demo classes.

Demo classes are your choice if you're a P/T (if you're not a real teacher, shut up and do as you're told). If you're F/T follow the contract. 

If you decide to do it then don't complain about it, either do it or don't. 

Demos should be about 15 minutes. 

I actually like them, 80% success rate. If a student is serious about looking for a school, it makes a difference.

But the problem is the school will often use the best teacher to do the demo but the cheap teachers get the students after that.