The school Kanji 校 introduced in level 7 is said to contain the radicals lid and father, but later on in level 25 a mix radical 交 is introduced, and used in four other Kanji. This feels very inconsistent, would it not be better to introduce the mix radical in level 7 as well? It just feels wrong to click on the mix radical and not see school listed.
This happens a lot, and you will continue to see it more and more as many radicals are just old Kanji. For example, they intruce the cloud radical after 会. I think the philosophy of it is isn’t super helpful for a mnemonic, they will put it off until it becomes relevant. You can always just make a mnemonic on your own for it, but it’s easier to remember a weirdo father messing with you school rather than associate mixing with a school.
Yeah it’s like megazerox said, kanji that pop up frequently as a radical in other kanji are introduced again as radicals. This is to keep with the flow of kanji being made up of radical building blocks. Of course, this doesn’t retroactively alter previous mnemonics, only for kanji going forward. I guess in the 交 kanji case, wk didn’t feel the need to make it a radical until a sufficient number of kanji using it cropped up (level 25), by which point it already passed 校 (level 7) and so the 交 radical doesn’t have 校 listed as one of its kanji.