In one of his post-season interviews. Even the interviewer thought Petruzzelli would hit UFA if he didn't sign by the summer, but Yzerman stated that because he wants to finish school, Wings retain the right for another year. This exception is only for players finishing school though, which is why I guess it isn't well known as most players jump at the chance to go pro asap.
Here is the ruling on it:
(i) If a Player drafted at age 18 or 19 is a bona fide college student at the time of his selection in the Entry Draft, or becomes a bona fide college student prior to the first June 1 following his selection in the Entry Draft, and remains a bona fide college student through the graduation of his college class, his drafting Club shall retain the exclusive right of negotiation for his services through and including the August 15 following the graduation of his college class. The Club need not make a Bona Fide Offer to such Player to retain such rights.
tl;dr: Wings will retain his rights until August 15th of next year