Hi @rezamary ,
Welcome to the community!
You may try to check https://www.youtubekids.com/ see if this will help. I have not yet tried this but I heard it is one of alternative way to filter youtube for kids.
Cheers! 
@rezamary wrote:
How can I have YT filtered during school hrs (9am-3pm)? I know I can URL filter in parental Control but that is not what I want. Just YT to be unavailable during certain time of the day.