I know someone whose dad is David, whose brother is David, whose married to a David (who has several friends and relatives called David) and whose boss is David.
Also, working in schools, I can assure you that ANY naming scheme you choose for users will inevitably invite a double-entry because of the way probability works - even in a school of only a few dozen students.
And where you come up with a smart-alec user naming scheme, there will always be doubles or you'll have to add a number to the end because of exactly the same problem. Every single place I've ever worked has had a duplicate username identified by, say, "username2" or similar, and the same problem with every influx of new users.
And, worse, if you use a smart-alec scheme like "first four letters of surname followed by first two of first name", you will invariably end up with something approaching a rude-word and it will ALWAYS be the most senior person in the place (so, obviously, you are then asked to explain and to change the naming scheme or provide an exception which breaks all your user scripts). I actually have to audit pupil names for pronunciation and possible connotations before I issue users (very easy to get swear words, insults and I guarantee you the pupil with the most fanatically religious parents will end up with something blasphemous under the naming scheme).