A sestina is a strictly patterned poem consisting of six six-line stanzas and a final triplet. It will take you a while to write a sestina for English class — you'd better start writing!
The form of a sestina is so complicated it can feel like a math problem to compose one. Sestinas have a total of 39 lines — six six-line stanzas followed by a three-line envoi, or brief stanza. The last word of every line is repeated in a specific pattern, so the six words are rotated through the endings of every single line. The earliest sestinas were composed in the medieval language of Occitan, during the 12th century.