What is there about schoolwork that should point to the gospel? Their schoolwork is sent to them by God! Schoolwork is a wonderful opportunity to teach your kids how God is involved in the most intimate details of life. Therefore, schoolwork becomes an opportunity to call your children to faith in Christ.
So, when your children are not motivated to do schoolwork, your first step is to encourage them to trust God. He didn't make a mistake. He actually ordained that they would have this work to do at this time. Encourage your children to trust God's wisdom in providing their work for them.
God has not made a mistake in giving the assignment, even if the human teacher did! Encourage your child not to rely on his own feelings and understanding, but to actively trust God's wisdom. The ultimate reality is that your child’s assignments are from God. (Colossians 3:23 teaches that this is true for all tasks, not just schoolwork.) Therefore, this is a matter of obedience to God — schoolwork is not primarily about getting grades or pleasing teachers and parents. Schoolwork is about bringing honor to God.