Pay more, or offer enough other perks to entice employees. Teachers already have a massive perk in the fact that they work ~1/3rd fewer days than other jobs. That's a great draw, if you can offer good enough compensation to get over the "not making enough money to live a comfortable life" threshold.
If people will risk their lives as cold-water crab fisherman for the right price, there's certainly a price to put up with highschoolers.