It took me a while to find the best way (well that's merely my opinion) to do this. Some posts and threads were saying delete the school or delete the bus and such but with the Nraas Career Mod you can actually enroll your Sim child or teen into 'Homeschooling'.

You can get the mod here: Nraas Career Mod

When you click on the Town Hall (active as a child or teen) there are the normal options but with this mod there is also the option to 'Enroll in Home Schooling'. Like with any option at the Town Hall, your sim will go there to register. And then it's done! A notification will come up to tell you that your Sim is now registered in Homeschooling and that they will receive daily homework they need to download and then complete as normal. This way they still receive homework that can alter their grades and as per normal, not doing the homework by the next day can cause a drop in their grades.