By the entrances of most the school buildings (i.e Girls' Dormitory or Library) there are flowered areas, just walk up and it should say pick flowers. Press to pick a flower. You can only hold ten flowers at a time so go up to a girl (or boy) and give them the flowers, then repeat until you've picked 50.
**You can check how many flowers you have picked:**
Start---> Stats---> General---> Objects & Items---> Flowers Found