Well, Minecraft does save changes on the fly, not when player leaves the map. So you need to have some backup and when the world is unloaded, replace old map with your backuped one.
For Spigot/bukkit servers there are some plugin tools (used especially for "automatic mines" or minigames, AFAIK MineResetLite can be example, or WorldEdit with Schematic files, or Map Reset), but for vanilla server/singleplayer you need to use commandblocks and area replacing - for that you have to have two copies of area and replace one by another via commandblock, but that really depends on area size (and expects closed area of course), for bigger ones it can heavily overload or crash the server...
And I really don't know how you deal with multiple players starting at same time? Do they share the area?
If I would want to achieve your needs, I would aim for something like "daily resets", because that would be easier than reset after every player leaves the map (and more server friendly).
That can be achieved by script replacing map files from backuped folder on server restart (that you still do daily, don't you?), however you need to be sure players leave the area or they might get stuck.
With spigot/bukkit you can try to find some plugin for it, there are probably some around.
Best Answer
This is possible however would require editing region data in your minecraft world. It is possible using something like NBTExplorer to copy and paste the compound tag containing all the data about the entities from each chunk into the same chuck in the new world. This would require you to manually copy and paste each chunk's entity compound tag into each chunk of your new world but if you are willing to do that then here is how you would go about it.
First you would need to download a tool for editing the NBT data in your world. Personally I use NBTExplorer which you can download here
Once you have downloaded an editor open up your minecraft saves folder inside the editor. Then you will need to navigate to the chunk you wish to copy. For instance here I found the chunk at 0,0 and have opened that.![enter image description here](https://i.stack.imgur.com/Wg8sC.png)
Then you will need to navigate to the compound tag which contains the entity information and copy it as shown.![enter image description here](https://i.stack.imgur.com/EsUcN.png)
Then navigate to the same chunk on the new world and paste it as shown.![enter image description here](https://i.stack.imgur.com/eEzlv.png)
Finally the new chunk should look like this if all worked. The process will need to be repeated for all chunks that need copying. You will also need to click the save button once finished.![enter image description here](https://i.stack.imgur.com/w22Bf.png)