It's not compulsory to give a gift to your teacher at the end of the year, but it's a way of thanking them for all the work they've done for our children throughout the year.
It's also a way for children to express their gratitude for having listened to them, supported them and helped them throughout the school year.
For them, the mistress is on a pedestal! She's a hero, the one with the power, the knowledge, always benevolent... and never changing her tune! At least once in a parent's life, everyone will hear "yes, the teacher said it! And there's no point fighting it, because you know in advance that the mistress will win!
My mistress will love it!