You could try blocking his anime website on your home network. Then he can keep using the Chromebook for school work but can't access his favorite sites.
If schools are going to provide these things, they should have the sites the kids might need to access white-listed and block everything else. Telling parents to try and block things is not realistic.
Other parents are going to fight that though because it's a free pacifier for their kids.
Parenting involves doing a lot of things that will probably be wildly unpopular with your children if you are taking your responsibilities seriously.
Remember, as a parent you have a specific role. That role is not to be a friend to your child, but rather to be a parent. Making these roles clear with everyone involved is what makes it possible to parent effectively and still be on good terms with your child.