Edit: (I should have read the earlier posts before writing this since it's redundant)
nope needed to be said more than once.
When I opened my school there was another school that had just recently shut down, within the previous 3 months. I had no affiliation or knowledge of the other school but as it turns out we had extremely similar curriculums. So to the point. After being open maybe 3 months a kid walks in says he's a black belt from this other guy that just closed down, he's real serious about his training and will have no problem bringing in his certificates to prove it. We talk, he knows the curriculum (by name) and moves well enough so I tell him he can wear the black belt and he will work towards his 2nd when the time comes. Well on the mats he only remembered about 1/3 of the material so I quietly pulled him aside and told him again that the requirements are the requirements, keep the black belt but your 2nd degree test will consist of everything white to 2nd degree. So he trains with me for about a year maybe a little longer, the whole time "I can't wait till I get my 2nd, my 2nd, my 2nd....." His old instructor opens up a few towns over, this gentleman gives him a call next thing you know he is promoted to 2nd and I said nice knowing you......
No matter what belt they wear some are your students for life some just until they get what they want.