Why would you not be a good teacher? [She's not even sure there is such a thing.]

A sacrifice is always something from the body of a person. [Though not the caster, specifically.] It can be hair, blood, bone, skin, teeth, nails...Glen also had an umbilical cord in his supplies, although I never found out what kind of magic that casts.

[And even if she had, she wouldn't have been able to cast it without the proper contract with a spirit.]