Please help! I updated to El Capitan and since then I cannot install some fonts into the system. I drag them into the fonts folder and it just won't let me, and it doesn't even appear a box telling me anything.
Yep, thats the new imitations (amongst others) forced by System Integrity Protection. Fonts don't need to go into the /system/library so shouldn't go in in there.
You have to put them into /Libary/Fonts or /Users/[Name]/Library/Fonts. Alternatively, just open and import them with the Font Book application. That way they will also be verified for errors.