If you want to begin a sentence with but, you need no comma after it. If you think beginning a sentence with but is too informal for legal writing, that’s fine. I say you’re missing out on a vigorous, forceful transition word. But my point here is that if you’re going to use but to begin a sentence, you need no comma.

Not these:

• But, such a ban would violate the students’ fundamental rights.
• But, the presence of alcohol would raise the same concerns.
• But, an action for contribution against a responsible party can arise in other ways.

But these:

• But such a ban would violate the students’ fundamental rights.
• But the presence of alcohol would raise the same concerns.
• But an action for contribution against a responsible party can arise in other ways.

The only exception arises when a comma is needed for some other reason, as when you need a pair of commas for a parenthetical insertion:

• But, the plaintiff later realized, the money never went to her sister.