Paris — Trembling, Tanya Riahi waits outside Lycee Gabriel Faure for the exam to begin. Since April,the 18-year-old Tanya has been reviewing, and for the next eight hours, she will pour out all her acquired knowledge writing essays on subjects like ''Why defend the weak?'' and ''Is it reasonable to love?''
The ordeal, which 270,000 other high school seniors here are now experiencing , comprises one of the most sacred and imperious of French institutions, the baccalaureat. ''Le bac,'' as it is called, is a national obsession. The French word for cramming is ''bachotage,'' and the exam divides France into two camps, ''bacheliers'' and ''non-bacheliers.''
Passing the exam assures entrance to almost any university. But unless followed up by a degree or specialized diploma, the bac does not assure any job. Still, failure leaves a stigma for life, a second-class citizenship socially as well as educationally.
Traditionally, the bac capped the tough mental grounding of a high school education stressing the humanities. Until reforms in 1965, even science students were force-fed rhetoric and deduction in their nine hours of mandatory philosophy a week.
Charles de Gaulle's technocratic, scientific vision of the country shifted the priorities for the bac. The most prestigious of the different subject combinations for the exam used to be the so-called ''option A,'' emphasizing literature and philosophy.
But today, ''option C,'' with its heavy doses of math, science, and economics , has become the pride of the elite. It can lead to the scientific or administrative grandes ecoles, similar to elite American graduate schools, such as L'Ecole Nationale d'Administration, and a career as an engineer or high-level ministry official.
In addition, a technical baccalaureat was introduced under the C option in order to make up for a shortage of middle-grade technicians. Nonacademic children are steered into technical high schools where vocational training leading to the new practical exam replaced the theoretical emphasis of the traditional bac.
The institution of this technical bac, along with the growth in academic high schools, has led to a boom in the ''bachelier'' elite. In the past 20 years,the number of 18-year-olds receiving the bac has increased by 31/2 times, according to the Ministry of Education. In 1960, only 11 percent of all French 18 -year-olds passed the exam. Last year the figure was 28 percent, or 242,531 students.
''When I passed the bac in 1939, all the students were from the upper classes ,'' explained Jean-Jacques Gelier, a rector at the prestigious Lycee Condorcet. ''Today the situation still favors the child from a wealthy family, but there is more of a mix.''
Mr. Gelier approves of this democratization, but like many teachers he fears it has led to lower standards and misguided values. He dislikes the trend toward science and the Socialist plan to end the grading distinction of ''very good'' and ''good.''
''We used to be the only country to create a special class of philosophers,'' he said. ''If you didn't know Montaigne, Voltaire, and the others thoroughly, you were lost. Today the bac has become easier, more imprecise.''
Apart from the technical bac, however, the exam remains a rigorous, brain-searching exercise, much more trying than the equivalent English A-level exam or American achievement tests. In written sections held last week, students were subjected to three days of intensive, multi-hour tests in math, natural science, history, literature, and, of course, philosophy.
''It's grueling physically,'' Tanya said. ''I've been working morning, noon, and night for weeks now, and now these three days. I'm exhausted.''
She is also nervous. If she does not pass -- like one-third of the candidates -- she will have to repeat her entire year at shcool and take the exam again next spring. And she will be humiliated in front of her peers.
But even if she passes, she says the bac is ''worthless'' because it only assures her a place in an overcrowded, and not highly regarded university system. To gain admittance to the grande ecole of her choice, she will have to pass another, more competitive, exam -- ''le concours'' -- next year.
So much pressure is useless, she says: ''I don't feel the bac crowns my high school education.''