EDIT: Apologies. Updated with a link to what gorhill REALLY said:
Manifest v2 uBO will not be automatically replaced by Manifest v3 uBOL[ight]. uBOL is too different from uBO for it to silently replace uBO – you will have to explicitly make a choice as to which extension should replace uBO according to your own prerogatives.
Ultimately whether uBOL is an acceptable alternative to uBO is up to you, it’s not a choice that will be made for you.
Will development of uBO continue? Yes, there are other browsers which are not deprecating Manifest v2, e.g. Firefox.
I think it’s very unlikely that they would pay for the IT department to install Firefox on every Chromebook. You’re talking 14,000 students in this county and only the kindergartners don’t get Chromebooks.
You might be surprised! This type of change is usually automated and centralized, so an administrator shouldn’t ever have to even touch any of those Chromebooks. Might be worth having a chat with your school administrators.