Steer clear of netbooks, there far to small. If your going for a laptop i would say no smaller than 15" anything smaller and its a nightmare when typing plus there isn't that much screen real-estate. If you have the money i would go for the iPad option.
As colerainehigh said they dont display flash content but most sites now our moving away from flash to html5 etc which is compatible with the iPad as for usb amazon sell a dock connector to usb/card reader, we bought one and its pretty damn handy.
If you go the iPad route you'll need a mac and also an iPad charge/sync trolley would be handy if there school owned.
You'll need the mac so you can use the Apple Configurator tool to image all the iPads