Children in foster care have suffered some trauma and/or abuse in their life, that is what has caused them to be placed in care. This places great obstacles for the children and foster parents to overcome.
These children are not like the children in your family. I will try to explain or hit a few key points.

These children have a lot of anger about most everything going on in their lives when coming into care. The foster parent is the one in the line of fire. The anger may not have a thing to do with you, you are just the person that is there. You are standing in the spot they want their bio parent to be. Their bio parent could have been sexual abusing them, beating them, and breaking their bones. They still are their parents and children love them (even if they aren’t loved back). They may see their bio parent once a week, they can’t be mad at them. Don’t expect these children to feel good or be grateful to have a safe place without pain, food to eat, clothes to wear, someone at home to care for them, etc.

School in most cases are difficult for the children in care. If they go to school, it’s not on a regular basis, mainly they are behind. Their parents didn’t care about school. The child is now living what they have seen. You are the bad one since you make them go to school. You expect them to do their work at school and you make them do their homework. The big hated thing, tutoring and possible resource (special education classes), to help get them on grade level.
I had a third grade foster daughter that couldn’t read and needed a lot of work on her phonics. I talked with her teacher about her reading difficulty. She received special help during reading with someone (one on one) and she received tutoring (2 days a week a hour each time). Yes, in her eyes, I was the Wicked Witch for a while until she started learning to read, then was successful in school. Then I wasn’t so wicked, just a witch, but a step in the right direction.

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"And in the end it's not the years in your life that count. It's the life in your years."
Abraham Lincoln