NBC will bring in a new cast of characters for its live-action sitcom “Saved by the Bell” and has renewed the show–its top-rated Saturday-morning program–for next fall.“Saved by the Bell: The New Generation” will feature a new cast of freshman students at fictitious Bayside High. The only returning character will be the role of the school principal, played by Dennis Haskins. A network spokeswoman explained that the characters for the current series were getting too old to continue playing high school students and were eager to pursue other projects. Peter Engel, who developed “Saved by the Bell,” will exec produce the new series through NBC Prods., the network’s in-house production unit. The original “Saved” premiered in September 1989 and is currently playing both on NBC and in strip syndication, distributed by Rysher Entertainment. The network is keeping open the chance of crossover appearances by the original cast and recently aired a prime time “Saved by the Bell” movie that drew a very respectable 9.5 rating, 17 share, ranking third for the week among teens. NBC went to an all live-action Saturday-morning lineup this fall coming out of a Saturday edition of “Today,” dropping animated kidvid fare.