Yuri Yuri Panic: Toutosugiru Jian ga Hasseishiteimasu!

The most beautiful girl in my class is asking me, a plain-looking loner, to marry her!? Akari, an ordinary high school girl, suddenly receives a filled out marriage certificate from the popular Luna Kiyosumi on her first day at school…? With the help of her imaginary friend Omochi, a stuffed animal with only yuri on his mind, this is the start of a precious girl-meets-girl school romantic comedy!

(Source: Comic Action, translated)