When you ask your pupils, students and colleagues "did you have a nice Christmas" when you return to work – consider stopping and listening to the answer they give. How many of those you ask will simply be putting on a brave face, or providing an answer they feel is 'the right one'?
If you are asking the question – are you prepared to continue the conversation if the answer is not what you expected? For example, how would you continue the conversation if the response was similar to "it was a horrible and lonely time and I considered completing suicide"?
If that response would make you uncomfortable or even quite scared, you could support them further by directing them to your workplace Mental Health First Aider – perhaps consider becoming one yourself if you don't know who they are or if you don't have one currently?