Wikipedia
Mesivta (also metivta; Aramaic: מתיבתא, "academy") is an Orthodox Jewish yeshiva secondary school for boys. The term is commonly used in the United States to describe a yeshiva that emphasizes Talmudic studies for boys in grades 9 through 11 or 12; alternately, it refers to the religious studies track in a yeshiva high school that offers both religious and secular studies. The comparable term in Israel is Yeshiva Ketana (, lit. "small yeshiva"). After graduation from a mesivta, students progress to a beis medrash, or undergraduate-level, yeshiva program.
In practice, yeshivas that call themselves mesivtas are usually a combination of mesivta (high-school) and beis medrash (post-high-school) programs. Students in the beis medrash program are often called upon to mentor those in the mesivta.