VFR on top means flying above a solid cloud layer, but in clear air.
On the one hand it is very comfortable. Sunny, normally no turbulence and everything, on the other hand navigation based on ground features is, well, sort of impossible so you have to use radio navigation or GPS, and you have to have a plan to get down eventually. Of course, without an IMC or IR, you cannot legally descend through the layer so you have to be pretty sure the layer eventually opens up.
And you need to think about an engine failure, which will force you to descend through the layer (temporarily being in IMC), possibly without the use of your artificial horizon (depending on its power source - usually vacuum) and with no way of knowing what's below.
If the school in Greece is CAA approved, it would show up on the CAA standards document eventually. I don't have time to check the link right now but it sounds very promising.