As a retired teacher, I may say that presents, if they weren't obviously within the range of a child's pocket money or obviously made by the child, always made me feel uncomfortable as the more expensive ones can feel like bribes.
Any really expensive gift was declined by my colleagues and myself and the head-mistress was prepared to state that the school had a policy against staff receiving expensive presents.
My best present, was the parents of a pupil, who when enrolling their younger child said "Oh, how nice to see you, does this mean our daughter will have you for English? We've been crossing fingers that she would."
So stick to thank you notes or letters, and perhaps, a potted plant or a bunch of flowers. No one could consider that a bribe for better marks.
And in case you are curious, I have had pupils and once parents ask me, what it would cost to get me to give their children better marks. You should have seen the look on the faces when I said "It would cost me my job, and both you and I a prison sentence, as the laws against bribery are strict in this country."