The Jedi would most likely never knowingly allow somebody to train in the Dark Side within their temple. This answer is mostly expanding on Richard's point that the Jedi and Sith have competing values.
Here are Mace Windu's thoughts on the Clone Wars and the Dark Side:
Spoilers for Shatterpoint
The Seperatists are not the true enemies of the Jedi. They are enemies of the Republic. It is the Republic which will stand or fall in the battles of the Clone War. Even the reborn Sith are not our enemy. Not really. Our enemy is power mistaken for justice. Our enemy is the desperation that justifies atrocity. The Jedi's true enemy is the jungle. Our enemy is the darkness itself: the strangling cloud of fear and despair and anguish that this war brings with it.
Note that the jungle Windu is referring to was on a planet that was steeped in the Dark Side because of lots of war and killing. In this jungle, the Dark Side drove Windu's former padawan, Depa Billaba insane and she fell from the light. In short, the Jedi's true enemy is the Dark Side itself.
Here are the views of Darth Plagueis:
Spoilers from Plagueis
For while toppling the Jedi Order and the Republic was essential to the task of restoring order to the galaxy, that goal belonged to the realm of the ordinary--to the world that was nothing more than a byproduct of the eternal struggle between the light and dark forces, both of which were beyond any concepts of good or evil. The greater goal of the Sith involved toppling the Force itself, and becoming the embodiment of the galaxy's animating principle.
Further down (talking about ruling the galaxy and other stuff):
But not until the singular flame of the light side was extinguished from the galaxy. Not until the Jedi Order was stamped out.
So as you can see, if Darth Plagueis tried to waltz into the Jedi Temple and declare that he wanted to study the Dark Side there, I think Mace and the other Jedi would have a problem with that.
What about the Dark Jedi?
Let's take a look at a few prominent Dark Jedi:
Asajj Ventress, Aurra Sing, Set Harth, and Depa Billaba were all Jedi that fell to the Dark Side. Ventress and Sing were responsible for the deaths of many Jedi. Harth was responsible for the deaths of a couple civilians. Depa was charged with crimes against civilization because of her actions on Haruun Kal.
Suffice to say, it is improbable that the Jedi would allow any of them to study at the Temple at all.
There are many other Dark Side practitioner's as well, but they most likely wouldn't get the nod either.
- Darksiders tend to not like the Jedi.
- The Dark Side can corrupt Jedi.
- The use of the Dark Side is generally incompatible with Jedi beliefs.
- I highly doubt Coruscant and Korriban had a foreign-exchange program for students.