I thought last I heard it wasn't an option for Edu accounts.
I've been waiting for them to make it available for some time. I'd love to see it just included with Edu (paid, staff) accounts, but I'm not holding my breathe on that.
I'm fine with them not allowing individual purchases until they put in proper administrative controls and settings for it...I just hope they do that sooner rather than later.
I have YT Premium for personal use - and I always forget how terrible commercials are on YT when I happen to see a teacher user it - or when I'm logged into my Edu account.
I know they have a couple things they can do to mostly get around the ads (without being a flagrant violation of TOS) but it still seems less than ideal