There are different methods but it depend on what you want to do with a single SSID and many VLANs.
1) single SSID to assign different roles (=topology, policy, ACL) to clients
This will allow you to seperate the mobile clients and also assign different roles i.e. student VLAN#10 & could only access the internet, staff VLAN#20 and could access everything
You'd do that with RADIUS as mentioned before but I prefer to use Netsight NAC.
2) single SSID with different topology (VLANs) to have more/smaller broadcast domains
There is a new feature in v9.21 that allows to create a topology pool (which include many topolgies) and clients get assigned i.e. round-robin to a different topology
-Ron