difference between ´subjunitivo pretérito imperfecto and condicional simple´

[JayfromLiam]

Supuse que no se retirarían los alumnos antes de que tocaran el timbre. 

Supuse que no se retiraran los alumnos antes de que tocaran el timbre. 

I want to know if there is a difference between the meaning conveyed by these two sentences. 

In the first sentence, ´se retirarían´ is condicional simple. In the second sentence, ´se retirara´ is subjuntivo pretérito imperfecto.

Can someone tell me? Thank you very much.