Here's the idea I'm trying to convey: "He was by far the least proficient in his class."
Here's the sentence I want to use: "He was the (slacker) of the group." Except that "slacker" sounds a bit wrong for what I'm trying to say. Is there a better noun that would fit here?
"underachiever", "boat anchor", "doorstop" all have varying degrees of negativity and rudeness but what might also play a role in your choice of word, of course, is whether this person's performance has any affect on you and the class. If the person is just not trying and is lazy, as slacker implies, a derogatory word such as "gold bricker", "dead-beat", "goof-off", "loafer", "slouch", "sloth", "back-slider" or "lazybones" could also work. (per www.thesaurus.com)
What about deadbeat, idler or loafer
Slacker implies laziness, so that may or may not be a good word for what you want. If this is a matter of ability rather than motivation, then I like Kris' suggestion of incompetent instead.
Another word you might consider is inept. The words unskilled or bumbling might work, too, but they suggest a shortcoming in dexterity more than intelligence (such words might work well in a trade school, though, for the situation you describe). If you want to emphasize results (or a lack thereof), you might try unproductive.
Yet another way to express this sentiment is to forgo the synonym, and instead opt for one of the many catch phrases used to humorously describe the shortcomings of individuals in relation to their peers, such as:
Not the sharpest knife in the drawer.
Not the sharpest tack on the board.
Not the sharpest tool in the shed.
Not the brightest crayon in the box.
Not the brightest bulb on the tree.
although some of these might be considered trite.
You might say “He was hindmost in the group.” Hindmost means the most to the rear. Or say “On a good day, he was last in his class.”
If you want to laud a person's effort, even if he fared badly in a course, you might say something like “In spite of being worst-prepared and most-disadvantaged at the outset of the course, and in spite of always faring last, he stuck with it and was not among the many who flunked out.”
You could say he was the laggard of the class, the class laggard, or (slang) that he brought or brings up the rear.
He was the _____ of the group. (least proficient)
Besides the suggestions of laggard and _incompetent, I would also consider one of:
I think that some additional context would have provided the OP with more signal and less noise. What class/group are we talking about? A proficiency for what exactly? etc.
You could also say that he was the weakest in the class if you're wanting to not state strongly as to why they're in that position. In the same vein you could say that they were the least competent or worst ?
You could call him a couch potato, clock-watcher, lazybones, or a loafer.
Gold brick is another term that is informally used in AmE.
