Let f:mathcalXrightarrowY be a morphism from an Artin stack to a scheme such that f is an immersion. Then mathcalX is automatically an algebraic space, so we're done by Knutson, Algebraic spaces, II.6.16.
Additions prompted by Brian's comment
Assume that f:mathcalXrightarrowY is a schematic map, and that Y is a scheme; then f is the pullback of f over the map of schemes mathrmidY, so mathcalX must be a scheme. Knutson needs lemma II.6.16 because he doesn't use the now-standard definition of schematic, but atlases instead.
When using immersion, I always mean jcirci, where i is a closed immersion and j an open one, following EGA I. But I understand that this is not a better choice than the other way round, and that they are only equivalent when the morphism is quasicompact.
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