Show them that school is important. If you can't go, get an adult the kids respect and ask them to go for you. I understand that parents work crazy shifts and can't always be there. If you aren't available, get someone who can help. Kids need that support.
Actions speak louder than words.
Especially in junior high, remember your kids are growing up. Give them age appropriate freedom but be the adult, not their friend. They have friends. They don't need you to be one too. They need you to place limits.