Some vs Certain

Certain and some define things as either specific or unspecific. But, they can refer to the same number. Certain apples are red. Some apples are red. Pick only the red ones.

"Certain students" adds to the meaning of "some students" the fact that the person making this statement knows which students they are but is not volunteering the names.

definite but not specified or identified; "set aside a certain sum each week"; "to a certain degree"; "certain breeds do not make good pets"; "certain members have not paid their dues"; "a certain popular teacher"; "a certain Mrs. Jones"