I took AP classes and in fact I'm currently doing 2 university summer courses. Like summer assignments, the workload can be big and in university, saying you have to care for kids isn't a valid excuse. HOWEVER, you're not in university, you're in school and you shouldn't be bearing such immense responsibility. Your parents may be unaware you have to do assignments because as you said, you've been laying in bed doing nothing, so they may think you can put your time to good use. If you tell and show them what you need to do for the assignments, how much work it is and you cannot care for the foster kids at the same time, they may understand. Also, tell your parents you would like to spend time with the foster kids (even if you don't) simply so they don't view what you tell them as an excuse to not care for the foster kids and instead hang out with friends.