The reason Wikipedia is not reliable is that it is basically a battleground. Especially in controversial or high-profile articles, there is usually a roughly equal number of people working hard to compromise their integrity and push a certain POV vs. the people trying to enforce Wikipedia policies, and even when the latter are winning, Wikipedia policies don’t help too much in the face of widespread consensus among a mostly-liberal mostly-atheist editor population. I have seen a lot of good editors come and go because they don’t know how to navigate the massive bureaucracy behind the scenes or they just felt marginalized and persecuted by the others.
Wikipedia is founded upon verifiability, which means that any given fact or assertion can be looked up in a secondary source and corroborated, but so many editors contribute information that it is hard for the small minority who comb through and watch for obvious errors or dubious material that needs to be flagged or removed. So the end result is that a well-watched, uncontroversial article, particularly one that has risen to GA or FA status, these can be a goldmine of reliable information, but once you reach into small corners of the site looking for stuff on a particular project, you can be hit with some terrible stuff. My editing career there is almost like a part-time job and even when I take a deliberate break from it, I still find lots of errors to correct as a gnome as I casually browse articles that interest me.
I wouldn’t totally discourage Wikipedia use for school projects, but I suggest always going for the cited facts and always following up by clicking those citation links and reading what they really have to say. I often do this as the References and External links sections of Wikipedia articles are highly concentrated good information. Good luck.