Sakashima's Student

If the chosen creature is a token, Sakashima's Student copies the original characteristics of that token as stated by the effect that put the token onto the battlefield. Sakashima's Student is not a token.

Sakashima's Student copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below) and it is a Ninja in addition to its other types. It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

If the chosen creature has {X} in its mana cost, X is zero.

Because you return the unblocked attacking creature to its owner's hand while activating the ninjutsu ability, you can't have Sakashima's Student enter as a copy of that creature.