Grantaire almost certainly falls in with Bonington and Huet on hating Gros' atelier.
What happens in an artist's atelier? The apprentices copy. And copy. And copy. They copy at the Louvre. They copy at home. They copy at the workshop. Their copies get picked apart by the artist. They copy some more. Eventually, they might actually get to help fill in the background of a work the artist is preparing for the Salon or for a state commission. And then they copy some more. They copy in pencil, they copy in ink, they copy in oil. They can try out compositions and the artist will pick those apart. They will copy paintings of still lifes for ages before any fruit is set in front of them; they will copy paintings of naked women, then statues of naked women, before they get their first chance with an actual model. Many cheap prints of major works were made to be sold to art students so they could have something to copy at home. Generally, after four years of this, of being immersed the work of the artist running the studio and the stuff on view at the Louvre and maybe the modern art collection at the Luxembourg Palace, they are competent enough to begin entering compositions of their own for the Salon and maybe enter into competition for the Prix de Rome, where the government finances the winners to live in Rome for a few years to do more copying and a lot more original painting because they'll have less supervision.
Bonington and Huet got bored out of their minds very quickly with all this damned copying. Notably, both of them became very important landscape artists. They wanted to be, and needed to be, out in the fields sketching from nature, not copying the crap that passed for landscape painting in previous centuries. (The French conception of landscape painting prior to the huge Romantic flourishing of the genre was for idealised landscapes; the English conception was for recognisable landscapes. It's not to say that the English did not idealise their landscapes but that the whole point was to depict a real place, with all the feelings of a real place. The French were more into entirely fake places.) Gros, a history painter, was patently not the person they should have been learning from, so it's no wonder they ditched out.
However, let us also not forget the politics of the art market in France at this time. This isn't England. In England, private exhibitions and private galleries existed. The market wasn't dictated solely by the annual show put up by the Royal Academy. A man who was not well-liked at the Academy show had other venues to display and sell his work. These other venues did not exist in France at the time. There were art dealers, of course, but they didn't hold public shows of their paintings for sale, meaning one got exposure only to already interested private buyers, not public recognition by name and style which helps develop ones commercial prospects. The men who juried the Salon were, you guessed it, those famous and great artists running major ateliers for students. So the only viable access to the market, which was predominantly the state, was through those patronage ties one developed with the man one apprenticed to, unless one had other connections in the art world or in the government.
So with what we know of Grantaire's personality, I think any talent he had didn't matter a damned bit. He likely screwed himself over, either accidentally or on purpose, just by being himself. I can't imagine he took to anything structured around endless copying, nor can I imagine he'd keep that to himself, and he'd certainly let it affect his prospects for any support of getting a painting into the Salon. (if you have him expelled in 1824, keep in mind the Salon was that autumn - plenty of ways he could get in deep shit with Gros and with everyone who hoped to submit, so this could actually be really fun and exciting.)
A geographic question: if he's from Montauban, why do you have him at school in Marseille? Montauban is essentially on the other side of France and is a major centre in its own right (26,000 in 1825 is a big city). It has a collège communal and feeds into Toulouse, from obvious geography, since I cannot figure out if the collège communal is first or second rank (first rank confers the exact same prep for the bac as a collège royal but without the boarding option; second rank schools don't offer all the necessary courses but one can start there and transfer up to a better school). It also, by 1826, had a school of drawing. [Source: Dictionnaire de la géographie politique et physique de la France et de ses colonies
If you have a reason for it, fine. It just really jumps out because so much is regional at this period that unless government work sent them there, it seems really, really unlikely that a family in the Tarn-et-Garonne would have anything to do with the Bouches-du-Rhône. If you're going to send a kid away that far, and not to the nearest major centre (in this case, Toulouse), you might as well just send them to Paris unless you have really, really compelling family reasons.
What kind of literature and what kind of life is the same question. - Tom Stoppard