The Lycée Chaptal, formerly the Collège Chaptal, is a large secondary school in the 8th arrondissement of Paris, named after Jean-Antoine Chaptal, with about 2,000 pupils. It was taken over by the City of Paris in 1848 after the founder ran into financial difficulties. The pupils were expected to go on to careers in commerce or manufacturing. The curriculum was innovative for its day, with emphasis on French rather than classical studies, and on modern languages and science. At the first it was primarily a boys' boarding school, but it is now a co-educational day school. The present buildings were completed in 1876. Notable alumni include Alfred Dreyfus, André Breton, Jean Anouilh, Daniel Hechter and Nicolas Sarkozy.

Prosper Goubaux (1795–1858), a writer and professor of the University of Paris, had founded the Pension Saint-Victor in 1844. It provided board and lodging for students at the Collège Bourbon. Goubaux saw growth in industry, commerce, agriculture and applied sciences, and saw that parents wanted their children to be prepared for these occupations through special studies. However, contemporary state education ignored these needs and was solely concerned with classical literary studies. Goubaux wanted to create a college for the sons of the prosperous middle classes, from whom would come the heads of the great commercial and industrial enterprises. It would teach boys to understand their times, and to appreciate the great achievements of modern civilization, while also being aware of literature and the arts.

The idea of vocational education, and of replacing study of the classics with courses in French, modern languages and science, was revolutionary. At the time, most learned men thought that a classical education gave a solid, moral basis. Without it the only end in life would be to make money. Goubaux asserted that models of virtue and heroism could be found outside the poems of Virgil and Homer, and that the study of science and of all of creation was more valuable than the study of Livy or Tacitus. In France, surely public education should also include the study of France, the French language and French literature. Abel-François Villemain, the Minister of Education, said "A French college in France, never!"

The city of Paris proved more open than the state to the ideas advanced by Goubaux, and allowed him to open the establishment called at first the École municipale Francois I^er, changed to Collège Chaptal in 1848. Courses began on 7 October 1844. The school was supervised by a board composed of the director, Prosper Gobaux, and six members of the municipal council. It was located on a site between the Rue Blanche and Rue de Clichy. The school did not conform to the normal pattern of state schools. In its lower, or preparatory classes it gave elementary primary education. In its middle classes it gave advanced primary education, and in its upper classes it gave scientific secondary education. There were also elements of classical secondary education, since it gave some Latin classes.

Goubaux ran into debt when a partner absconded with the school's funds. In 1848 he persuaded the City of Paris to take over responsibility for the school, while he continued as director. The study of Latin was introduced in 1850. That year the 6th year program was introduced, designed to prepare students for entry to the grandes écoles such as the École Polytechnique, École Centrale Paris and École des Mines de Paris. Subjects included technology, industrial economy, law and administration. In 1853 the courses were codified and defined in detail. Latin was definitely established in 1853 to meet the needs of the baccalauréat of sciences. Pupils were required to study that language if they wanted to continue beyond the fourth year.

Goubaux died in 1858 and was succeeded by M. Moujean, who directed the college until 1887. It was during his tenure that Chaptal became firmly established as one of the leading public educational establishments in France. By 1867 the school had 1,055 pupils. In 1868 the educationalist Matthew Arnold wrote that the college had 1,000 scholars, of whom 600 were boarders and paid £40 per year. It was one of the two great municipal schools of Paris, the other being the École Turgot. The École Turgot was a day school, mainly for the sons of small tradespeople, while the Collège Chaptal served children of more prosperous parents. An 1868 description said,

With a large staff of well-paid professors and teachers, whose cost amounts to nearly 12,000l., this school now pays a surplus of over 8,200l. per annum to the city funds. Among its professors are some of the most eminent in France, and they teach carefully and examine closely. For the six weeks preceding their vacation, the boys of the upper division pay visits to industrial works, and take notice of the dimensions of the machinery, and of the parts of the works, from which they afterwards execute plans and elevations. Some of these drawings are accurately made and beautifully finished. The boys from this school are almost uniformly successful in obtaining admission to the École Centrale des Arts et Manufactures, and a fair proportion of them pass the unusually difficult entrance examination of the École Polytechnique.

The original premises were irregular in plan and too cramped for the student body, despite additions and rented properties on the Rue Pigalle and Rue de Clichy. In a session at the Hôtel de Ville on 10 July 1862 the college's board of directors agreed on a move to "spacious gardens" of 13,500 square metres (145,000 sq ft) in the area between the Boulevard des Batignolles to the north, the newly opened Rue de Rome, the Rue de Bernoulli and the Rue Andrieux. The architect Eugène Train (1832–1930) was selected to build the new school following specifications defined by the city. Construction started in 1866, but was interrupted by the Franco-Prussian War of 1870. Work resumed in 1871 and was completed in 1876.