It all boils down to the Chinese custom of building relationships, 'gao guanxi'. Of course in this case, gao guanxi would produce some really unfair situations; teachers who receive expensive gifts from parents, are expected to take better care of these parents' children, to the detriment of those who don't offer gifts or who offer lesser gifts.
Yes,receiving gifts is corruption, as teachers are in public service, they should be treated as 'flies' that President Xi Jinping promised to swat!
To be honest,please don't think teachers are saints,they are normal.So sometimes teacher would have received gifts from parents,and take special care to their children.Always,most teacher prefer to like some students than others in spite of not receiving gifts.It is natural.
I have friend who is a teacher in China one year the Province had a very good crop of apples and they were very cheap to buy.Many parents gave my friend boxes of apples as gifts until her apartment was full of boxes of apples.I suggested she give some of the apples to other teachers,her reply was their apartments were also full of apples and they would be very angry if she gave them apples.I said if you like someone give them 1 box if you dislike them give them 2 boxes of apples.I was laughing very loudly every time I thought of a new use for her apples she soon became very angry with me.After several months the apples started to smell,I suggested she make wine with them she went crazy.