About address allocating

[Haisang Wu]

hi, I have the following questions about address allocating:
  I know SLA is /48, and interface ID should be 64 bits, 
does it mean that the smallest unit when allocating address is /48?
In other words, if I allocate a /48 to a large university, could I 
allocate a /48 to four middle schools, thus each middle school gets
an block less than /48, which is /50. Is this plan reasonable?
  And, could those who get sTLA of /35 give me some detailed plan in 
address allocating? Thanks.
  best
                       CERNET    Wu Haisang