You don’t specify the age or gender of the classmates (you do, however, say “all my classmates”), so depending on the context/type of school you could say:
“That's only because he is (a/n) “[no-talent/untalented] hit with the ladies (from ‘Graces Movies’ under 17. Alex Pettyfer) (for settings where the classmates are all females of all ages)
(note that the lack of talent here would have to be limited to his singing talent; otherwise using across-the-board "no-talent"/"untalented" with "a hit with the ladies" would be an oxymoron, in my opinion);
or in another context:
““That's only because s/he is a “no-talent teenybopper”
or
“… because s/he is a/n ““[no-talent/untalented] hit with [the] teenyboppers" (from Google Books and Wikipedia)
(primarily in settings where the classmates are adolescent girls, but in spite of Wikipedia’s definition, adolescent boys can be “teenyboppers.”)
For a phrase that could work for all friends and acquaintances of all shapes and sizes (and which wouldn't necessarily require adding "no-talent" or "untalented" because that notion is already built-in), you might say:
“That's only because s/he is a lightweight (alone as a noun)/lightweight pretender (as an adjective with ‘pretender’) with a damn/ed good agent/who must have a damn good agent.”
Lightweight-noun: “3. A person of little ability, intelligence, influence, or importance.” (from ‘The American Heritage Dictionary’)
Example use of “lightweight pretender” from ‘Drive’ by Clayton Lachmund via Google Books)
Example use of “must have a damn good agent” from a random ‘Whisper’ user’s reaction to Ben Affleck getting the role as Batman.
Please note that all of the above could be preceded by “nothing but/nothing more than” for emphasis (as in the example from ‘Drive’) but it wouldn’t be necessary.