Both sentences are correct, but there is a very slight meaning difference about who is most responsible for the action, and I do mean very slight. Without the apostrophe, junior xyz is taking full credit for his action. With the apostrophe, the school is taking some credit for the action, because in that sentence, it seems that the apostrophe is added to emphasize which school the student is from; it's no longer the act of an individual, as he is now being associated with the school.
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