Munki (cost-free and open source) is a way to deploy software to and update software on a fleet of Macs. It’s used widely by schools, universities, and corporations (here’s a list of only what’s publicly known about who’s using Munki, and that’s only what can be easily found publicly).
Even if you have the money for a cheaper MDM like Mosyle (which is fully featured at a very affordable per-device-per-year price but also has a one-platform-only free tier), it really operates quite differently from Munki, and ideally you would have both together. Mosyle even has a write-up on this: Why deploy Munki using an MDM solution and Apple DEP.
Many schools use and like Jamf, which is an MDM but also has a lot of non-MDM features. Even with Jamf, there are those who desire Munki’s features and deploy the two together. To get a sense of why, you might want to read this thread: Software Patching – What’s the future?.
If your school cannot afford an MDM of any kind (please don’t use Profile Manager as your MDM), Munki on its own can be an affordable (just costs time to set up) and extremely useful tool for managing Macs (you’ll just miss out on certain things that Apple requires a user-approved MDM for—as of this writing, that’s kernel extension approval in 10.13.4+ and tcc in 10.14+).