When the countries stand together, they party. Hetalia Gakuen is the place where all the cute characters gather together to plan the ultimate party. During school days, you need to get to know the characters, the more affection they have for you, the more chance they will accept your invitations to a "date" in the holiday mode.
But the most important event is the party at the end of the month. Realize the strengths of the countries. If Japan is in charge of the food, you'll get Japanese food and if England is in charge of the music, you get a rock concert. Besides the 8 countries, you can date the less featured countries, the ultimate purpose is have fun.