See the SEP entry about Pythagoras : from ca. 570 to ca. 490 BCE, and the entry on Pythagoreanism :
Pythagoreanism is the philosophy of a group of philosophers active in the fifth and the first half of the fourth century BCE :
Many other sixth-, fifth- and fourth-century thinkers are labeled Pythagoreans in the Greek tradition after the fourth century BCE. [...] There are nonetheless a number of thinkers of the fifth and fourth century BCE, who can legitimately be called Pythagoreans, although often little is known about them except their names. The most important of these figures is Hippasus [from Metapontum fifth-century BCE].
But we have to recorc also :
Aristoxenus (ca. 375- ca. 300 BCE) is most famous as a music theorist and as a member of the Lyceum, who was disappointed not be to named Aristotle's successor. In his early years, however, he was a Pythagorean, and he is one of the most important sources for early Pythagoreanism.
This shows us that the school was "alive and well" during Aristotle's time (384–322 BCE).
Thus, I think it is not correct to speak of "collapse of his school".