Suugakuteki Renai Kakuritsu

Mizutama no Koigokoro

Noko is the serious and responsible class rep. Yuushi is a classmate who liked to poke fun and annoy her. She thought she would never have anything to do with him. Until one day when they found a baby left at the school with a note saying "This is Yuushi's child."
 
Yuushi said, "If this child was entrusted to me, then I will protect him!" Even though Noko thought he was a jerk most of the time, she was touched by his declaration and she blurted out, "I'll help you take care of him, too!" When it comes to the baby and Yuushi, she will come to love them both!
 
(Source: Shogakukan, translated) 
Included one-shots: 
Volume 1: Zettai Ouji Renairon, Suugakuteki Renai Kakuritsu 
Volume 2: Mizutama no Koigokoro, Defense Bakari no Koi Ja dame!!