Ron Rosenfeld wrote:
Actually, if your student is already rated and current, i.e.,
can act as PIC, you do not even need a medical certificate
at all -- see 14 CFR 61.3(c)(2)(viii).
The CFI would still need to have a medical certificate if the "rated and
current" PIC student is "under the hood" (14 CFR 91.109(b). In that case,
the required safety pilot is a required flight crew member and the
exception in 14 CFR 61.3(c)(2)(viii) would not apply.

Indeed. However, the required safety pilot need not be the CFI,
and you could still teach a student under the hood -- legally
and without a medical, with a properly rated safety pilot on
the right seat (in which case you have three people on board
who can log PIC time simultaneously! :-)

--Sylvain