Yes they can literally be on any subject. Cake baking 101, Forensic Science, How to make an airplane. Obviously the better the class the more chance it has of being added as a paid class, very basic classes may be listed as free (you still earn 5 coins / $5 for free classes). Our preference is great classes users can learn things from that aren't just available anywhere on the internet.
When you add a class you can suggest the cost (Free, 2 Coins, 4, 6, 8 or 10) ultimately we have the final decision though. This is for everyones safety, we don't want users adding small bad classes for 10 coins and students getting ripped off. We do take your recommendations for class cost highly though.
Interesting site, I'm a teaching on another website but will check yours out. Do you offer online skype classes?