Keep things in perspective - it is never wrong to give a demo class when asked for it, but it's always wrong to work for a crappy training centre, especially when asking their teachers for a deposit!

Spending free time giving non-paid promotional classes is absurd! Only a fool would would spend down time in scholastic situations in which they they have nothing to gain while others may gain at their expense. If you give free demos you are an i_ _ _ t! A demo class should be paid at a higher rate than normal as it is an inconvenience.