Did Mr Moore and the latter teacher, Billy, ever meet in an episode? What happened to Maria, and the other leaving class members before the newer members took their place? Can anyone give me all the info to fill me in?

Maria went to an arts school (I think she got a dance scholarship?) and Janice went to Harvard, but I'm not sure about the others.

I think Jawaharlal and his family relocated to California. And Mr. Moore left to pursue his acting career. I don't know if he and Billy ever met.

Nope, they never had an episode where Charlie and Billy met.