Most Roman slaves never learned to read or write, those that did were either taught by another educated slave, or their master sent them to a Gramaticus (school) to learn. Also some slaves were teachers.
The trade route of rum slaves, sugar and molasses was called triangular trade because it was the name of the merchants who exchanged them during this era. Learn the harsh treatments experienced by the slaves by visiting.
It depends on what time period and location you mean. Some were encouraged to do so. A slave was allowed to learn as much as his master needed him to know. Giving slaves too much education was discouraged, as uneducated slaves were easier to control.