In a nutshell Sister, the schools of fiqh were established within the first three centuries of Islam by four great Imams; basically fiqh is the expert interpretation and deriving rules of Quran and hadith by scholars qualified in such matters, and since the four Imaams understood some of the evidencies differently and reached differences of opinions in a minority of rulings, this is why the schools are considered different schools named under the founding Imaam of each school
hope this helps
salamualikum wr wb