Self - Teaching an Intermediate 2 alongside Five Highers? Watch
My advice is get the spec for the subject and work through it point by point making sure you know all that's required. Personally I found the standard textbooks a waste of time and just got decent revision guides as they tend to have the concise what you need to know points that match the spec instead of a lot of babble and extra 'of interest' material that might slow you down.
Any school can enter you for the exam and the NABs but most likely to let you is your own school I think, otherwise you'd have to find somewhere else.
but if your school gives you the nabs, material and the exam and your focused i see no reason why it cant be done mate
Also, do you have to sit the NABs?
Yes you have to sit the NABs.