In what Dewey numbers are most of the high school library's literary criticism books located?

It depends on how big the school library is but generally literary criticism falls under:

809 Literature - History and criticism, (Literary criticism), (Evaluation of literature), (Books - Appraisal), (Criticism and interpretation)

For more specificity, such as poetry:

809.1 Poetry, American poetry, Canadian poetry, English poetry, Love poetry, Children's poetry, Humorous poetry, Religious poetry, War poetry, Poets, (Verse), (Poems)