Complex sentence: Students who want to attend the school must apply in their sophomore year of high school, although some exceptions have been made-some freshmen have successfully become Mass. Academy juniors in what would have been their sophomore year at their "sending" school.
Split into simpler sentences: Students who want to attend the school must apply in their sophomore year of high school, although some exceptions have been made. For example, an occasional freshmen has entered the Academy in what would have been their sophomore year at their "sending" school.