It may be of interest to know that the Register of Professional
Archaeologists has established "Guidelines and Standards for Archaeological
Field Schools" and does maintain a certification program for field schools.

http://www.rpanet.org/members/CertificationStandards.html

Unfortunately, to date there have been few institutions that have applied
for certification. I do not even find a list of those that have been
certified on their website, nor have I ever seen a field school advertised
as being RPA certified, so I would not judge it to be terribly successful
thusfar. The fact that the information is buried in a "Members Only"
section of the website might have something to do with it.  The porgram has
not been around very long, however, and perhaps participation will grow.