This gives an approximation of the difference in the teacher's XP gain for "level at once" and "many levels at a time" teaches. The "approximation" is due to the fact that it assumes each level costs the same xp.
Number of Levels: for XP
All at once gives approximately: XP to teacher.
One at a time gives approximately: XP to teacher.
A teaching fail gives approximately: XP to teacher.
Teaching one level at a time gives the teacher approximately or % more xp.
Teaching all at once reduces the teacher's XP by approximately or %.
Failing the teaching skillcheck reduces the teacher's XP by approximately or %.