If the students spoke their native language, they would be punished in lots of different ways. Some of the most common punishments were isolation from other students, withholding of meals, and washing their mouths with soap.
The children had to branch out and try new things by being forced to speak a language not familiar to them. It helped them communicate with people that they encountered later and maybe even work for.
The children were forced to speak the English language rather than their native language when they already had to leave their home. It probably made them even more unsure and scared than they already were. In addition to that, they were losing the native language of their people and some even forgot it.