This is a really sweet couple! Yes, he is a teacher, and she is 10, but she is obviously in love with him, and he did buy her an engagement ring! Fujitaka was 11 years older than Nadesiko, and that worked out beautifully! Terada-sensei deals with it very well, and hardly ever lets his feelings show. He is very discreet about it, and obviously intends to wait for Rika to mature before taking any further steps.
Rika is actually very mature for her age (everyone says so), and it is natural for girls to like older guys. (Sakura and Yukito, for example) It is also pretty common for school girls to have huge crushes on handsome young teachers. Rika is also very discreet, and apparently only Tomoyo is able to guess who it is that Rika likes.
Their relationship is so sweet!! When Terada-sensei enters class and sees Rika, generally he will blush! Rika just smiles beautifully...(sigh) Rika likes to help him with his work or his papers, and buys him special presents for holidays. And then there was the Christmas when he came to see her with that little box in his hand.....(how romantic!!) but nothing else has happened. Terada is a very honorable guy!
See pictures of the happy couple!