In the Adachi Ward Junior High School Number 12, a student was admonished recently for appearing one day with an Elvis Presley-styled, duck-tail haircut. It was "undignified," his teachers said.

The boy responded at first by cutting classes. Then he returned to school one day, sassed the teachers and began kicking them severely until he was brought under control.

It was a more or less typical example of a wave of schoolroom violence that has terrified teachers, alarmed police and parents, and provoked a vigorous round of academic questioning about what has gone wrong in Japan's junior high schools.

Other incidents have been more violent, involving 14- and 15-year-old youths who have turned on teachers with steel pipes, bamboo swords, fists, belts and toy-model pistols. Many assaults are carried out by groups. After being warned to stop playing a tape cassette in class, nine youths at one school swarmed into the teachers' quarters and beat several with bamboo swords.

Police also cite the case of six teachers who were beaten and struck by leather belts after they had scolded four teen-agers for extorting money from their classmates.

The episodes puzzle authorities because Japan's junior high students are usually obedient and respectful toward teachers, whose strick rules govern all aspects of their conduct.

It is not clear whether there really is an increasing number of such violent incidents or whether simply more are being reported to police and the news media this year. Last year, about 800 students were taken into the juvenile court system for abusing teachers, double the number of four years ago, and police say that a far larger number of cases go unreported because school officials fear bad publicity.

About 90 percent of the offenders are boys in junior high school, police statistics show. The three-year junior high schools include students aged 12 to 15.

Most of them are from families low on Japan's economic scale and most also are academically backward in the classrooms. They are collectively dubbed "the droputs," although they usually continue to attend classes while not participating much.

Educators and police blame an assortment of causes for troubles, including poor discipline at home, the demise of neighborhood controls in urbanized Japan, violence on television and in comic books, and the influence of older boys who have been absorbed into Japan's criminal gangs or motorcycle hot-rodders' organizations.

But a number of educators blame primarily the school system for offering nothing of interest to the backward pupils whose poor performance in the lower grades excludes them from the competition to get into college. They say that school instruction is geared exclusively to the student striving to pass the tough college entrance exams, which largely determine success or failure in the Japanese economic system.

"It is the present system of school management," asserts Tsunekazu Takeuchi, professor of education at Kokugakuin University in Tokyo. "They start separating out at a very early age those who have a future and those who don't.

"The school just concentrate on exams -- it is the only factor they consider. But those who are not going on to college must live in that environment, so it is natural that they would have inferiority complexes."

Around the age of 10, Takeuchi added, a Japanese student knows from his grade reports whether he has a chance of getting on the college-bound track. In primary school, he is ranked each semester with his classmates in a five-tier grading system that sharply separates the successful from the backward.

At the junior high level, Takeuchi said, the poor student is faced with three years of classwork that is directed toward college exams and is thus totally irrelevant to his life.

In an older Japan, the distinction was not so harsh because there were respected careers for the poor students. In modern Japan, where the respected careers are in business and government, the uneducated must settle for the least respected jobs.

The immediate trigger for school violence usually is a teacher's scolding for infractions of rules, and many educators agree that the rules are far too strick for the times. All public school students must wear identical uniforms -- girls wear longish skirts and boys dress in stiff military-style tunics and trousers.

Administrators are traditionally strict in enforcing rules, and stories of teachers striking students are common, despite a natonal education law that prohibits physical punishment in schools.

In his recently published book, "We Struck Our Teachers," Yuji Ikue, 34, described interviews with many young offenders.All of them, he wrote, had been struck at one time or another by their teachers, and none of them regretted having struck teachers themselves.

Police suspect a certain amount of planning and coordination in the attacks. Takeshi Suetsune, superintendent of the National Police Agency's juvenile section, goes so far as to say that "usually the attacks are planned in advance." A student angered by a scolding provokes another one by misbehaving and then turns on the teacher with a weapon. Most incidents occur in the present of the assailant's classmates, who rarely intervene and who often encourage the attack.

Suetsune also believes many assults are carried out by school gang-leaders, called bancho , who are in turn influenced by older gangsters or hot rodders.They have been known to extort money from classmates and give it to the older gangs.

Shinsaku Nju, who has taught Japanese language classes at the Adachi Ward school for 21 years, said he has watched the violence grow and widen in its targets in the past few years. At first, it was directed only against weaker victims, women teachers or elderly males. Now it is directed against the toughest placed in charge of disciplinary problems.

At one time, he said, some students holding positions of influence would quickly report the misadventures of their unruly classmates in an attempt to cooperate and quell violence. Not any more.

"Now, the colleagues never try to stop a friend's attack," Nju said. "Now, they goad him on, and he cannot stop."