It's stupid to ban students from using Wikipedia- sure, Wikipedia isn't of uniformly high quality, but it can be a pretty good encyclopedia. It's just not something you're allowed to cite.
If a school doesn't want students to read Wikipedia at all they really should provide an alternative encyclopedia that the school thinks is high enough quality for students to use, I think you can get subscriptions to Encyclopedia Brittanica now? But that costs actual money.
Actually, it's probably pretty smart for schools to "ban" students from using Wikipedia, in order to encourage them to develop habits to use better things. If you let them use Wikipedia for their research, you're putting them in a situation to slouch into using it for most of their research (except for some source laundering at the end).
The thrust of the link here is that they aren't giving them alternatives, and just telling students to throw themselves into Google and hope they find something. Which, yes, isn't a bad skill to learn either- there's stuff to find out there- but it's setting them up for failure.
There are hundreds of unpaid volunteers at all times prowling for and reverting vandalism. The most popular articles are next to impossible to change. And to top it all off, if a large amount of vandalism happens on one article, it just gets reverted and locked for a while so no changes can happen, period.
Then anyone following can see what you saw.
Wikipedia is a moving target, so it could be a terrible source on a topic for the hour you're looking at it, and much better at other times. Trouble is, those other times don't do you any good. That inconsistency means it can't ever really be an "excellent" source.