It’s just like giving a hairdresser a tip or a waiter a tip - sure they get paid for doing a job but sometimes when you’re really pleased with how someone does their job you gift them something, be it a tip or a small token of appreciation. It’s totally optional of course and I don’t believe teachers expect it as such.I don't get an end of year present for doing my job, so why should they? Surely their salary is their reward for their work.