In the limited time available, there is little you can do. If they are at A2/B1 level, you are not going to get them up to B2, especially if they won't learn on their own.
If the school has aims for this type of volunteer help, someone should be giving you some suggestions. Is there no-one you can ask?
If not, console yourself with the fact that you are probably helping them a little.
- For Teachers