Best choice for avatar, where user data is sensitive?

I'm serving a web app where user data is pretty sensitive. An ID number and full name is the only personal data given. However, since users are all students and many will be managed by an administrator, I still want to give each user some kind of additional identifier.

Since we can't have students upload their personal photo as an avatar, I've experimented with avatars such as animals with color background, initials with colored background, and the student's "team color," which was used to quickly identify student grouping.

I liked the ideas of the animals (there were around 50 to different animals) and wanted to make some of them "unlock-able" to encourage use of the app (similar to Stack Overflow badges). Students and admins were confused upon using the app though (e.g. "why am I a chicken?"), but I'm considering running with it.

Any thoughts on an ideal solution? Thanks in advance for any ideas!