I'm volunteering at my kids's school, teaching coding with code club projects. Unfortunately, the school doesn't allow usage of internet by students. Another problem is that less than half of the class can bring laptops to school, which can run Scratch offline. Is there any way I can use Scratch on tablets (which most students have) while offline?

Not easily
You could bring your own laptop in and run your own fork of Scratch on it and then set it up as an access point and get the pupils to connect to it

But not really practical I'd thought

You could also search for people who want to donate/sell for a low price their old laptops to the school and use those for your project.
I volunteerd in a project called Coder Dojo and they had laptops for the children who couldn't bring their own. These laptops were donated to them or bought with profits from donations/merchandise sales. They ran Linux so that they didn't have to pay for an OS.