my iPad has just started dropping out of numbers, so when there's no wifi I lose access to the spreadsheet

The tags are selected by you when you submit your post.

Alas, an iPad9 is capable of update to iPadOS 16.7.2 - or optionally to iPadOS 17.1. If your iPad is still running iPadOS 15.4.1, it may be too far behind to be updated to the current version from iPad settings. Let's address this first - and if the problem remains, we can work through any residual problems using current software. Keeping your iPad updated is important; updates include both functional fixes and essential security updates.

If your iPad is capable of update to a more recent version of iOS/iPadOS than is currently installed, these support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

Be aware that while an iPad may be capable of update, in some circumstances you may need to use a computer to perform the update*. To do so, you will need access to either a Windows PC (with iTunes for Windows installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

(*) If you don’t have access to a suitable computer of your own, that of a helpful friend or family member will suffice. Alternatively, the technicians at the Genius Bar of your local Apple Store or Apple Authorised Service Provider (AASP) will be delighted to assist.

Genius Bar Reservation and Apple Support Options - Apple

If you have a beta profile installed, ensure this has been removed from your iPad - as, otherwise, installation of production software updates may be inhibited. This support page provides the necessary information to remove the beta from your iPad:

How to uninstall iOS beta software - Apple Support

You should note that for your iPad to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter. Similarly, the iPad can only be updated over a standard WiFi connection - or be updated by a PC or Mac using iTunes or Finder - over a USB connection.

The tags appended to your post suggest that you have a first generation iPad Air with iOS 12 installed.

First and foremost, this model iPad is now very old - considered "vintage" in Apple's parlance - and is now well beyond support. The most recent version of iOS for this iPad is iOS 12.5.7. Consider that supported models of iPad are capable of running iPadOS 16.7.2 or 17.1 - these being current/supported versions.

If you are using your iPad for critical applications - which, by your description, you appear reliant upon your stored lesson plans for your work - you would be best advised to consider replacement of your iPad with a current model. In so doing, you will remove many barriers in both identifying and remediating any technical problems that you may be experiencing, be they hardware or software related.

As for exploring your current issue, without access to a similarly old device, it will be very difficult attempt to replicate your current difficulty. At the very best, being optimistic, your iPad battery will be reaching the end of its usable life.