C-schools are advanced specialty schools which the US Coast Guard offers to their personnel.
Traditionally, military enlisted personnel go to boot camp to make the transition from civilian life to military life. Many times (but not always), after boot camp, enlisted personnel go to "basic job training" which is called A-school. After A-school, they usually will go to their specialty training called C-school.
For example, Bob joins the Coast Guard to become a radar technician & operator. He will go to boot camp like all new enlisted personnel joining the Coast Guard. After boot camp, Bob would go to A-school to learn about basic electronic theory. Once he finishes A-school, Bob would go on to C-school to learn about radar and maybe even a specific radar system – which will become his normal job during his Coast Guard service.
In the Auxiliary, we are afforded access to certain C-schools to support our specialties. Many times Auxiliary personnel are attending these schools with active duty Coast Guard personnel.
For more information on C-Schools, go to the US Coast Guard Auxiliary C-School Portal on the National Member Training site: http://cschool.auxservices.org/.