At this point, the reunion will include Travolta ("Vinnie Barbarino"), Lawrence Hilton-Jacobs ("Freddie 'Boom Boom' Washington"), Ron Palillo ("Arnold Horshack"), and Marcia Strassman ("Julie Kotter"). Other participants are still being added so hopefully that means that Gabe Kaplan ("Mr. Kotter") and Robert Hegyes ("Juan Epstein") will also take part.
I believe the last time there was a Welcome Back, Kotter reunion was as part of ABC's 50th Anniversary Celebration. For the 2003 event, Kaplan, Travolta, Hilton-Jacobs, Palillo, Hegyes, and Strassman all reunited on-stage. Travolta even briefly reprised his popular character for some shtick with Kaplan.
Here' the press release about the