مَرَرْتُ بِأَحَدَ عَشَرَ طَالِبًا — I passed by eleven students.
مَرَرْتُ: past verb plus first-person subject pronoun.
بِـ: preposition.
أَحَدَ عَشَرَ: fixed two-part numeral in the genitive position after بِـ.
طَالِبًا: accusative numerical specification; the preposition does not make the tamyīz genitive.
Key point: The final fatḥah on «عَشَرَ» is a sign of indeclinable form, not an accusative case ending here.