In addition to the Pythagorean school, cited in nwr's answer, there is an equally famous group of the ancient era, the peripatetics or peripatetic school, which was the name given to the Lyceum of Aristotle.
The peripatetics were a group of students that gathered around the personality of Aristotle, and took its name from the cloister (peripatos) in which they walked and held their discussions. They dealt with philosophical and scientific disciplines, that, of course, were not so distinct as nowadays.
In particular, from a scientific point of view, Aristotle and his disciples dealt not only with physics, but also with biology.
Even if physics, and his treatises collected in Physics, are the most well-known aspect of Aristotle's philosophical and scientific thought, in a subsequent period of his life Aristotle dealt with biology, and his biological researches were the fruit of long and precise studies he carried out, with the collaboration of his disciples, during the twelve years of direction of the Lyceum.
Undoubtedly, even before him [Aristotle] the Greeks had undertaken various inquiries of biological nature, but Aristotle provided a new approach to these researches [...]. We need only recall that, in his works about biology, he speaks of about five hundred animal species, many of them studied with anatomic dissections, drawings of organs, and so on.$^1$
His most important book about biology is Parts of Animals.
The peripatetic Theophrastus, the successor of Aristotle as director of the Lyceum, continued the naturalistic and biological studies of his master, in his treatises Enquiry into Plants (Historia Plantarum)
and On the Causes of Plants .
The successor of Theophrastus, the peripatetic Strato of Lampsacus, marked a decisive turning point in the course of studies of the Lyceum, stressing again the physical aspect rather than the biological one of naturalistic philosophy, coming to express the necessity of a rapprochement to Democritus. He attempted to reformulate Aristotle's physics theories on the base of Democritus’ atomism.
Several modern scholars consider Strato as the 'highest point of Greek physics': but, unfortunately, we keep of him too little to judge exactly the value of his scientific researches and the meaning of his reinterpretation of Aristotle’s heritage .$^2$
$^1$ Geymonat L., Storia del pensiero filosofico e scientifico. Vol. I, L’antichità e il medioevo (En. tr. History of Philosophic and scientific Thought. Vol. 1, Ancient Era and Middle Age)), Garzanti, 1970, p. 276. My translation.
$^2$ ibid., p. 279