Card Captor Sakura is one of the anime I grew up with, and it’s the one that led me to buy my first manga volume, so it will always be special for me, even when some things creep me out a bit.
If you’ve only seen the anime, you might remember that Sakura had three friends that were there to prove that the main character had a life, and to make the problems a bit more interesting. One of those friends was Rika.
In the anime, Rika had a very obvious crush on their teacher, Mr. Terada (for example, there’s an episode when she’s being controlled by a card and she’s attacking everyone, so they use an illusion to make her see something with the power to make her surrender, and what she sees is the teacher).
[From Card Captor Sakura episode 9. Screencap from the CCS wiki. Thank goodness everything has a wiki, otherwise I don’t know how I’d get the images I need.]
A girl with a crush on her teacher, no big deal, it’s a common plot. Now let’s talk about the manga, where those two are dating. Teacher/Student relationship? Oh, that’s a common plot too, enough for it to have a page on TV Tropes. Wait, did I mention she’s 10 years old? 11 at most?
The authors try to take away some of the creepiness by showing that they’re not doing anything beyond going out together, and having the teacher buy an engagement ring for Rika:
[From Card Captor Sakura chapter 9; art and writing by CLAMP. Scan taken from mangareader.net (I tried to find the translator, but the link was dead, which is a pity because they get all my respect for the note they added).]
No matter what they do, it still makes most people feel uncomfortable (I know, I’ve made the experiment of finding people who remember the cartoon but don’t know about this and telling them).