I want to stop my kid from watching YouTube videos of Minecraft during "school hours".

I have set up what I believe is a blacklist policy for "minecraft", but access to the videos is still available.

I don't want to blacklist "YouTube" as some of the remote homework that school is setting uses YouTube videos. (And adding "youtube" to the balcklist hasn't worked anyway.)

Any thoughts on what I might be doing wrong?

(Screen grab attached)