who is right?

I'm not a native speaker, but I would say that since "percentage", the subject of your sentence, is singular, the corresponding verb has to be singular too.

I believe the correct usage is "is", but "are" is becoming increasingly common.

If the subject was "good students" then it would be plural, therefore "are would be correct".
If the subject were "large percentage", then "is" would be correct.

Personally, i have no doubt the latter is the case, but i agree with #2.

I think that in spoken English either are acceptable, though "are" sounds more natural (even if on formal analysis it is incorrect). In written English, I'd use "is" or recast the sentence to avoid the problem, e.g., "Many good students are ..."

(Stylistically, "a large percentage" is objectionable, because it sounds more precise than it really is.)

I would say that the first example is correct. I'm sure there's some grammatical rule to back this up, but because a large percentage isn't going to be singular, then it is plural.

I remember reading it somewhere, but I can't remember where.

I think most native speakers would say "are". Although "is" may technically be correct, it doesn't sound right.

You're Dutch, right? That's the source of the confusion, I think. In Dutch only "a percentage is" is correct. Not so in English.

#4 OZinOH don't wish to be fussy but it should be "either IS acceptable", ie either one is acceptable.

It seems to me that it's rather difficult to analyse incomplete sentence fragments. Maybe we would make more progress by thinking about meaningful sentences.

So here are some examples using "are":
"A large percentage of good students are immigrants."
"A large percentage of good students are from small families".
"A large percentage of good students are glad they chose to continue their education and would encourage their younger brothers and sisters to do the same."

Not only do all of these sound OK to me, but I don't see how any of them would work with "is". So I am going to put forward the claim that "are" is both more natural-sounding and more technically correct than "is". I am open to being persuaded otherwise - but please illustrate with complete sentences!