Content/Parental Controls for Schools

[reformed-skiddie]

Hi, what do you think is the easiest-to-use FOSS system for parental / content control at the moment?

I have Edubuntu 23.10 installed on a set of computers in a very cash-strapped school here (some with 23.04 still to upgrade), and we would ideally like to install one program that can block inappropriate content across other programs (e.g. both Firefox and Chromium, and anything else that can load remote content), and be managed from the admin account such that an unprivileged student account cannot mess with it.
Otherwise we may need to subscribe to an expensive blocking system at the router.

I looked at the Parental Controls Ubuntu help page, but it is horrendously out of date, as some of the systems recommended there have broken links or are over a decade old!
On searching this forum for parental controls, all the relevant threads I saw were several years old too, so I hope it's ok to make this one.

Is there any easy-to-use current solution to that, like WebContentControl was supposed to be in 2010? ...or must we suffice with separate browser extensions only?
For comparison, Sophos offer a freemium desktop software solution to control this for ***dows and M*c, but sadly they do not offer a GNU version.

If using browser extensions, how can we protect them from tampering? For example, FoxFilter was recommended, which technically has that capability, but it runs on a freemium model that requires a subscription in order to password-protect the add-on settings for just one browser, requiring a higher fee for the multiple computers that we have, so I'd like to look at options that don't need approval from finance first, especially if it's on a per-browser basis.
Edit: it seems that FoxFilter is also available for Chrome and *dge, and as an iOS app, but perhaps not Safari on old hardware. I'll look more into this.

Thanks for your time!