Well, in short, the mother looks like she's 14, and the kid is "openly" in love with her - loli x motha-con. His teacher, too, is in love with her mother, and so the love rivals duke it out in classroom, as well as plotting the downfall of their common enemy, his father. The father has a secretary who is in love with him. But once she discovered that he has such a "young" wife she turned her attention to his son, who is now torn between his own mother and a naive, motorbike riding girlfriend. Later on, the class gets a new student who looks exactly like the mother, whom everybody falls for. Except that the student is a he.... 
Yagami-kun OAV has a good chance of being the only anime of which the production video (tagged along with some episode) is more interesting than the anime itself. And they weren't kidding about 'Low budget manga spinoff anime aimed at exploiting the existing fan base. 
Description from: <a href="http://users.skynet.be/mangaguide/au1069.html" target="_blank">http://users.skynet.be/mangaguide/au1069.html</a>

Yagami-kun's Family Affairs follows Y&#363;ji Yagami, a high school student with a problem: his mother, Nomi, looks very young and Y&#363;ji has a crush on her. To complicate matters, his high school homeroom teacher is also infatuated with her due to how young she looks. Nomi is oblivious to all of this and is head over heels in love with her husband, Y&#333;ji, and frequently displays this affection very publicly, which causes more embarrassment for Y&#363;ji. 
Y&#363;ji's high school friends constantly tease him about his "mother complex". When Valentine's Day comes around, though, Y&#363;ji receives a box of handmade chocolates. The name on card can be read multiple ways, the most common way being Masayuki Ikari (&#20116;&#21313;&#37324; &#30495;&#24184;, Ikari Masayuki?), a male name, so his friends tease him throughout the day until it's time to meet the person who sent the chocolates at the front gate after school. 
The girl who sent the chocolates (the name is actually read "Mayuki") starts meeting him after school so they can walk home together, and they eventually become an item. The remainder of the story follows Y&#363;ji and Mayuki as their love grows and as Y&#363;ji works to resolve his "mother complex" issues. 
(Source: Wikipedia)

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