I think chromebooks are great, currently looking at getting new one myself atm (gf has basically stolen my old one...). Especially good as a casual browsing, entertainment, and lightweight office tasks machine.
Just much less hassle than Windows laptops, and in general a better user experience imo, performance even on modest hardware feels good, and often have great battery life.
The only reason not to get one is if you know you'll need to use specific applications not available on chrome / no chromebook compatible app in the app store. E.g. If you need all the features of desktop Microsoft word rather than the online / app versions, or you need to use Adobe Premier or whatever. I'd have thought everything you'd need for 6th form would be browser based.
You can use Linux programs on chromebooks too, which gives you access to a wide range of software. I don't actually have it set up on my current chromebook as I only use it for browsing & entertainment. However I installed Linux on one about 8 years ago (used crouton) and it worked nicely. I think the processes are a bit different now though but from a quick Google there are a few methods:
https://uk.pcmag.com/linux/135719/how-to-install-linux-on-your-chromebook
Second hand prices seem a bit variable atm from what I've seen - some good value options, some way overpriced, but maybe worth having a look at anyway if you want to save some cash. Worth checking what the AUE date (no more updates - so may start to get security vulnerabilities, and may start to become incompatible with apps etc) though to make sure it still has life left in it:
https://support.google.com/chrome/a/answer/6220366?hl=en