Both Kyou and Sekai offer training the main hero with their relationships with their girlfriends.
Also, Kyou and Ryou fell for the same guy, just like Katsura and Sekai, and get hurt because of that.
Written by Shirobake and LiCha.
+ both from visual novel as one the available routes
+ same school romance setting
+ both 2 girls from both series have similar personalities, one is outgoing helping the other shy girl
+ similar meet up, date, and break up pattern
+ both picture Male MC as indecisive and pathetic
- School Days is far more dramatic and extreme than Clannad kyou-Hen
- while Kyou-Hen is shorter than School Days, the original Clannad is far longer and better than School Days