It depends on what you mean by "closed subgroup". If you mean a Zariski closed subset which forms a subgroup then the answer is no. If you mean a closed subgroup scheme, then the answer is yes. An example where you need to use the second definition is the Frobenius map . If we let act on through then the action is transitive and indeed is isomorphic to . However, unless is zero-dimensional is a non-trivial finite group scheme whose -points consist of just the identity.
Note however that is always quasi-projective even when is a subgroup scheme so all homogeneous -spaces are quasi-projective.
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