Incorrect assumption. You are basing your answer on the "recent" comics - aka the comics starting with New X-men #117 (Sept. 2001) featuring the vastly expanded Xavier Institute with multiple dorm buildings, etc.
Rogue-like: Evolution is not based on that version of the school.
Instead, this game is based on the cartoon X-Men: Evolution, which in itself is modeled after the previous 90's X-Men cartoon & the Jim Lee run of X-Men comics from the early 90's as well. Each mutant attending the school (under a dozen) was also part of and trained extensively as the combat team in Season 1 (whether they liked it or not). Season 2 introduced the "New Recruits" - students who weren't part of the combat team yet but were being trained by both "Mr. Logan" & "Ms. Munroe" (and later "Mr. McCoy") in not just controlling their powers but how to use them both offensively and defensively in the Danger Room and on school grounds (including in things like Search & Rescue, as seen in one of the Boom-Boom highlight episodes of Season 2). Student count was well under 20 in total still.
Even then, certain New Mutants effectively "graduated" to X-Men team member status, such as Iceman (who really stepped up-to-the-plate in Season 3).
Simply put, lazy was not tolerated. Each student exercised their asses off to the point of physical and mental exhaustion, as seen in the cartoon numerous times. Hell, if we go back into the older comics we'll discover that a key lore aspect of being a mutant was peak physical/metabolic capability (meaning moderate exercise like jogging would keep a mutant "ripped"), immunity from all disease & illness, etc. - unless their mutant ability had something to do with an "otherwise power" (such as Blob & Pestilence, to name two) or story (The Legacy Virus, Pow-R8). The Morlocks were a spin on this lore-wise, mutants who were "not pretty"...until it was eventually revealed that they were genetically altered "Mutates".
I haven't touched the comics in over 25 years, so current year politics has likely retconned all that.
Included is the final shot of the future student-and-faculty body of the Xavier Institute. Note that in the show the kid in bone armor (Spyke) & Boom Boom had left (I think Boom Boom came back when Mystique resurfaced but I can't recall if it was permanently or not), X-23 + Havok + Angel + Gambit + Colossus weren't part of the school (the later two were enemies), Wolfsbane & Jubilee were removed by their parents starting Season 3 (they had almost no lines or screen time in all of Season 2). That leaves 4 faculty members and 12 or 13 students. Not the hundreds in the more recent comics and what you're referencing.
Long story short: In X-Men Evolution (and this game which is based on said cartoon), everyone is fit as fuck and athletic as all hell.
