Maybe turning the hose on the people making out in the corner would help.
My theory is that they are there to contribute to a production. Maybe if they are that bored they should go elsewhere.
Theatre is not all glamour, as we all know.
Maybe they could be helping each other memorize lines while waiting at rehearsal.
I think it is cool that you want to try to keep them interested by introducing elements early- actually that is a good idea so they don't get slammed with everything all at once- probably helps techs go a little more smoothly.
Unfortunately this problem isn't limited to the High School level. You are not alone.
SJM