Did I err in saying to the student, "Are we not your brothers?" after he objected to the teacher's تحديد (determining/setting) of the curriculum for the class, stipulating that other classes should not be informed?

If the information about the exam locations was available only to your class teacher, then your colleague's demand that all classes in the school have equal knowledge of these locations was justified in order to achieve justice and equality among students. His demand bore fruit as the teacher informed everyone. Your blaming your colleague was not correct. If you did not transgress the bounds of good conduct in your anger, then there is no harm; but if you did transgress, then you should seek his forgiveness.