[K12OSN] refuse multiple logins

[Peter Hartmann]

At our school I've made a bunch of accounts lettered a-z with a common
password for use  the little ones.  They're too young to deal with
logins themselves so the teacher can login with these accounts easily
since they have the same password.  However, not all of the teachers
understand (or can remember) that the system doesn't like multiple
logins on the same account. Teachers are worst students! Is there a
way to refuse multiple logins by default?

Thanks in advance,
Peter