Searching for an older series that I read a few books to. All I can remember is that it was about 5 or 6 teens that traveled (or fell) through a portal to a different world, one with dragons. I remember they were able to travel back and forth to their world. I believe that one of the character's names was "Zerena" if I'm not mistaken. In one of the books they fell into a chamber of gold (riches) and encountered one of the dragons. Pretty sure the word "World" was in the title, but through all my searching I can't seem to find it.