Were a US school and currently use both methods. Our student email is first_last@school district and all of our students use 8 digit (random) numbers to login. I have to say from middle school on most kids know their ID # ... passwords of course are always forgotten. I am a believer that K12 students email should not easily identify them. I think we're heading towards some letters from their name and some numbers as their email name. Keep in mind this is the US where guns, kooks, and disgruntled ex-husbands flow freely. Kids under 18 need to be protected.