Bogged down in understanding the subject of the “meaning it would take …. ”

Question

What is the subject of “As well as they ever do, meaning it would take a miracle to make historians out of them,” in the following novel? And the subject of “meaning …,” for that matter.

I also have been wondering if “As well as they ever do,” means that sixth-formers are shaping up this year as well as at any time they have ever shaped up, and if this also implies they are among the best. If so, there seems to be a contradiction between the first half and the second of the sentence. Give me some advice.

The following is taken from the mystery novel “A Fine End”
The story starts here:

They returned to the drawing room, and when Andrew had joined them, David Sanborne said,

“Nice choice, the Handel, I believe that’s what the Somerfield choir is doing at Christmas this year — am I right, dear?” He glanced at his wife.

“Our Nigel’s hanging on to his soprano part by a hair, I’m afraid. We’re all praying his voice will hold another few months.”

“It must be frustrating for boys that age, being neither fish nor fowl,” said Winnie, her color still high. “And then just when they’ve got themselves sorted out, grown a bit of hair on their chests, they have to move up and deal with Andrew.”

“I do what I can,” Andrew said. “Vile, back-stabbing little buggers, most of them. Your son excepted, of course.” He nodded at the Sanbornes.

David Saborne grinned. “Sixth-formers shaping up this year, are they?”

“As well as they ever do, meaning it would take a miracle to make historians out of them.” He gave Jack a malevolent glance. “Montfort here is an amateur historian of a sort — why don’t you tell them about your interest in the history of the Abbey?”

Here are some characters appeared here and some information.

Jack Montfort: architect, amateur historian, forty years old, single, no family, 
               Scotland Yard Detective Superintendent Duncan Kincaid’s brother-
               in-law  
Winfred Catesby:thirty-six years old, single, priest 
Andrew Catesby: Winfred’s brother, in charge of sixth-form history class, 
Suzanne Sanborne:archdeacon (a priest of high rank in the Anglican Church who 
        works under bishop), Winnie’s superior, 
David Sanborne:Suzanne’s husband, physician,  

Winfred invites four people for dinner at the vicarage.

Andrew Catesby doesn’t get well with Jack Montfort.

Winfred and Andrew become particularly close after they have lost their parents quite young.

Everything has changed since Winnie has become involved with Jack Montfort.

Answer 1

The subject of the sentence in question is "sixth-formers".

It's important to remember that people don't usually speak in complete (or even correct) sentences, and characters' dialogue will often be written to reflect that. In the passage you quoted, Andrew is responding to David's question "Sixth-formers shaping up this year, are they?". In his response, Andrew omits the entire subject and verb from his response because they would be redundant. The implied subject is the subject of the question ("Sixth-formers") and the implied verb is the verb from the question ("shaping up").

If Andrew's response were to be re-written grammatically, it would be something like "The sixth-formers are shaping up as well as they ever do. It would take a miracle to make historians out of them." In the original quote, he's spliced two sentences together as well as omitting part of the first sentence. When he says "meaning" he's referring to the first part of the sentence. He's saying that his students never shape up well, and that his current students aren't doing any better than usual.