My MacBook Air won’t update and I can’t download excel for school.

The oldest operating system that Microsoft supports for new application installation from Microsoft 365 or Office 2021 for Mac is macOS Catalina. You will need to purchase an appropriate license. Once macOS Ventura is released this Fall, Microsoft will drop support for Catalina and require Big Sur (11.*). You would need a mid-2012 MacBook Air to upgrade to Catalina (10.15.7), and a 2015 or later MacBook Air to upgrade to Big Sur.

You can use Microsoft Office Online with a Microsoft account which may simplify any need to download Excel.

Otherwise, you can view the product description line from the Apple () menu > About this Mac panel, and compare that to the hardware requirements in How to get old versions of macOS - Apple Support. Use the Safari browser, and there are links to the full installers in the Mac App Store if you choose to upgrade your operating system.