JUSTIN BIEBER BUILDING SCHOOL IN GUATEMALA ... Without Shirt

Justin Bieber ventured deep into the Guatemalan jungle to help a group of poor villagers construct a badly needed schoolhouse this weekend ... and TMZ has the pics.

After Justin performed in Guatemala City on Saturday, he joined up with Pencils of Promise -- an organization dedicated to building schools for communities in need.

JB was choppered into a remote area outside the city on Sunday ... where a makeshift construction crew was already busy putting up walls for the new school.