Yes, one can indeed play for free till level 20 on retail WoW servers, however there is even better option - Pirate servers.
They are almost completely free (you just pay for stuff that isn't actually accessible in real WoW, like some mounts and other irrelevant stuff, besides for our purpose the game itself is quite irrelevant), and since they are on the edge of legal and illegal chances are they are even less monitored, because owners of those servers would use certain level of security against governments and things like that.
Also there is a shit ton of free to play games mostly of which lower quality if that is of importance and if it makes it more accessible, but I do think that 99% could run WoW just well enough on low settings, and another great thing about pirate servers we can play older versions of warcraft so it is even lower quality of the graphics in first place, and options of guilds and private chats like that are the same.
My opinion is either F2P games, or pirate WoW servers, cause it is both free and everyone can join, and I think they would never expect something like this going on somwhere like that...
What game (or server) will it be is only a matter of agreement.