NORTH BILLERICA – For two periods on Wednesday, the St. Mary’s boys hockey team had done everything but score in its Division 1 North opening round game with St. John’s (Shrewsbury).Showing a maturity beyond the youth on their roster, the Spartans didn’t panic and punched home two third period goals to escape the Chelmsford Forum with a 2-1 win over the Pioneers.Click here for a photo gallery.The No. 4 seeded Spartans will now face No. 5 Waltham, a 4-1 winner over St. John’s Prep, in the quarterfinals (Saturday at Chelmsford Forum, 5:40).”That was two quality teams battling hard for 45 minutes,” St. Mary’s coach Mark Lee said. “I am so happy for these guys. There was never one minute of panic out there.”The Spartans (14-8-1) had a 27-17 shots on goal advantage and nearly twice as many scoring chances as Shrewsbury, yet needed some good goaltending from Donald Hesse in the first 30 minutes to keep the game tied.”Hesse made some big saves for us but that’s nothing that we haven’t seen all year,” Lee said. “From the forwards right through our defense core, everyone came to play.”Fighting off a sluggish first period and a second where they dominated the play but didn’t score, the Spartans turned the light on first early in the third.Keeping the puck in the St. John’s end for 45 seconds, the Spartans finally were rewarded when Corey Bartlett’s bad angle shot hit the skate of goalie Sam Byrne and trickled across the goal line.”We really felt confident throughout because we were playing so well,” Lee said.Less than two minutes after Bartlett gave St. Mary’s the lead, the Pioneers responded as Devin Smith fed Brian Connolly in the high slot for a one-time rocket over Hesse’s shoulder to tie the game at 1-1.The Spartans kept pressuring and went on their fourth power play of the game midway through the third when Dan Duquette was called for boarding.St. Mary’s needed only 32 seconds to capitalize as Cam Kennedy’s blast from the left point was stopped but Julian Yourawski got to the rebound and poked it past Byrne for his sixth goal of the season.”Yourawski just never stops going out there,” Lee said. “He’s the same way in practice all the time. He’s like our Energizer bunny.”The rest of the game belonged to the St. Mary’s defense as it hemmed in the Pioneers. In fact, the Spartans’ puck possession was so good down the stretch that St. John’s couldn’t get Byrne off for an extra attacker until there was eight seconds left on the clock.”We didn’t panic when they made it 1-1 and kept on playing,” Lee said. “We played so well and the timing for that is good. This was a great way to start the tournament.”