Can a doctor draw blood from his Rebbe?
It is permitted for a doctor to draw blood from this Rebbe. The prohibition not to draw blood from a parent is based on the stringent prohibition not to hit a parent, which is a serious sin and is subject to capital punishment. Although we may not hit a Rebbe, or in fact any other Jew, we don’t find this stringent aspect regarding a Rebbe.
As a side point, although a child may not draw blood from a parent, this is only if there is no one else to do it. The reason being because essentially it is permitted to draw blood from a parent if it is medically needed. The reason we may not do it is only because we might accidently draw more blood than necessary. Therefore, when left with no choice, it is permitted. Regarding a teacher, the stringent aspect of hitting is not applicable, we are not afraid that a little more blood than is needed might come out, and it is permitted.
Chashukei Chemed Babba Metzia 33a, Ran Sanhedrin 84b.