Buying an iPad on Amazon
Amazon is an authorized Apple reseller. iPads whose product page says sold and shipped by Amazon.co.jp (Amazon.co.jpが販売、発送) are the same official Japanese units Apple sells on its own site, and the warranty goes through Apple too. The same page can also have third-party sellers mixed in, so with the slightly cheaper ones, check who's actually selling. Some are parallel imports or have been opened.
iPad currently comes in four lines: the regular iPad, mini, Air, and Pro. For online classes, watching videos, and taking notes, the regular one is plenty. If you want a better screen or want to draw or edit video, look at the Air and Pro. The mini is good for reading lying down and tossing in your bag when you head out.
You can add AppleCare+ when you order, or buy it separately later. The enrollment window in Japan is short: you have to add it within 30 days of buying, and after that it's too late.
iPads rarely go on sale on Amazon, but during big sales the ones Amazon itself sells occasionally get a sale price, and AppleCare+ is sometimes discounted along with them.