How do I explain blockchain using an analogy?
Use a classroom activity, then present that as the analogy.

An old campfire activity, for those that remember it. A growing story that nobody knows the end of, or even if it will end.

The objective is to create a story, with everyone adding their parts, in turn. Someone starts the story by saying a few lines, and ending mid-sentence, just before some action happens. The next person repeats what the first persons said, and has to finish that sentence with something that makes sense, and then continues the story, using their own idea, since they don't know what the first person was thinking. Like the first person, the second stops mid-sentence in what they're saying. The third person repeats everything the second said (which includes what the first one said), and finishes the sentence left incomplete by the second. Adding more lines to the story, this person also ends mid-sentence. It continues in a similar fashion with each person repeating the whole story from the beginning, adding a couple lines, and ending mid-sentence. At some point the "next" person will not be able to repeat, even in their own words, the story so far, and the chain fails, hence ending. If it happens to be too short, or if not every student has had the opportunity to participate, a new story may be started, and try again. Once everyone has had a chance to participate, in as many rounds as you deem appropriate, you can pick up the chain and finish the sentence, and the story, bringing it to a successful completion.

After the story, or stories, have run their course, you can relate the process to a blockchain. Each piece (except the first) depends upon the preceding piece, and is meaningless without it. If, at any point, someone in the chain doesn't hold up their part of the "contract," by completing the previous sentence, the story is ruined. Still, even without an end, everything up to the last incomplete sentence remains valid, and can be traced back to the original piece.

This is totally non-technical, so it will not be of value in discussing blockchain implementations. It will, however, be memorable, and the students should be able to grasp the concepts behind a blockchain implementation when you do present it.