When it comes to stirring up a little excitement in class, Otis Spofford knows just what to do. He can turn a folk dance fiesta into a three-ring circus . . . or an arithmetic lesson into a spitball marathon. Best of all, Otis likes teasing neat, well-behaved Ellen Tebbits--until the day his teasing goes too far. Now Otis is nervous, because Ellen isn't just mad . . . she's planning something