I haven't done this with the use of a NAS ID but I don't think you need to for what you are looking to accomplish. On the Aruba side your Auth Server should look something like this:
aaa server-group "802.1x-ServerGroup"
auth-server 192.168.1.30
auth-server 192.168.1.31
set role condition Class contains "students" set-value Student-Authenticated
set role condition Class contains "teachers" set-value Teacher-Authenticated
!
On your IAS/NPS server you should have 2 access policies. 1 for students, 1 for teachers.
-Student policy will have the windows security group for "students". It should also have the class attribute set to "students" as well.
-Teacher policy will have the windows security group for "teachers". It should also have the class attribute set to "teachers" as well.
Once that's done restart RADIUS services.
If this still isn't working look at your IAS/NPS logs.