By its very nature, Islam had to develop a religious class very early on. In Islam, there's a very rigid theology and a defined legal code that must be followed. During the lifetime of Muhammad, there were designated people who had the authority to pass legal verdicts (legal verdicts were in the domain of the religious authority). During the time of early Caliphs, religious teachers were sent to each new city in order to instruct the people about Islam.

As far as seminary schools, depends how you define them. You have the beginnings of a seminary in the Ahl us-Suffah who lived in the main mosque of Madinah during the lifetime of Muhammad and received personal instruction from him. Later on, you had "schools" which emerged around the companions sent to each city. These were more like semi-permanent study circles inside of mosques than purpose built seminaries though. I'm not sure when seminaries became independent buildings but because of their history, they've always been somewhat attached to mosques.