Mount second-five Pomare Tane charged down a clearance from the kickoff and showed good speed and strength to score to make it 10-5, but Leppard cancelled that out with a 50 metre solo effort to make it 15-5.
The hosts kicked a penalty to tighten the gap, but down on points and with the wind at their backs looked to have passed on a rear attacking opportunity.
Allen underlined that before the break when he proved too strong close to the line, as did impressive Mount prop JD Grant, who barged over from a quick tap to make it 22-13.
Excellent Bethlehem College defence and field kicking, along with the wind, meant the Mount never scored again, while Bethlehem added tries to Leppard, fullback Chris Johnston and lock Sam Waddell.
Bethlehem College First XV 42 (Matt Leppard 2, Thomas Allen 2, Diardu Terblanche, Sam Waddell, Chris Johnston tries; Jack Preston 2 cons, pen) Mount Maunganui College First XV 13 (Pomare Tane, JD Grant tries; Tama Taite pen)