A Bible school teacher switched his plea from not guilty to guilty in the sex killing of a teen-age girl whose body was found alongside a rural road near Novato. Scott M. Williams, 29, entered the plea just before a preliminary hearing was to begin before Municipal Judge Lynn O'Malley Taylor in San Rafael. Assistant Public Defender Paulino Duran said he was "shocked" by his client's decision. Williams, under a plea bargain agreement, pleaded guilty to one count of first-degree murder and two special circumstances--rape and a lewd act with a minor. He will be sentenced on Sept. 11. Williams, also a deacon and volunteer groundskeeper at the Bethel Baptist Church in Novato, was arrested April 19 and charged with kidnaping and murdering Jennifer Moore, 13, who disappeared April 13 when she left her home to buy an ice cream cone.