Usually the class mother collects money for the teachers and Rebbeim both at Chanuka and Purim. This is for a class/group gift. This eases the burden for those who can’t afford and the teacher/Rebbe gets a lump sum from everyone. Not everyone gives what is asked and some give more. Some give besides the group effort. Everyone tends to do what is comfortable for their own situation and what is nogeah between their child and his mechanchim. If one has a child who needs extra attention and that is noticed and responded to, then a parent might respond in a more generous manner to show appreciation at appropriate times. If a parent feels that no attention was paid to their child or a mechanech wasn’t especially caring toward their child their dissatisfaction might come out at this time as well. And if people are financially well off they may generously show their appreciation at times like Chanuka, Purim and Pesach knowing that the mechanchim are also on a strict budget necessitated by their salaries yet have the same expenses like everyone else.
So basically I would say to each their own.