First, there's a new iPad due out fairly soon (end of the month for the announcement, I think), but while it may be a lovely device, the original (which may come down in price as stores clear their shelves and stock iPad v2) has no USB sockets, which I see as a downside. (*)
You mention printing, but how much of that would be for your own benefit and not for the school you were in (ie would printing it be essential 'at that school' eg for an imminent lesson, or could it be done at home?)
Unless you know there are staff in every school you have visited who are using Apple kit, whether it be an iPad or Mac 'notebook' (sorry, lost track of all their names) then you could find yourself out on a limb regarding getting any assistance.
Internet access if not connected in via school wireless network could be done using 3G dongle, and the upper end iPads (600 to 900 for current iPads, I think) but a dongle and a laptop could be had for 450 to 500 quite easily and have bigger screen and keyboard, USB ports and capacity to plug into a projector etc.
Query regarding print was because I use a bit of software to 'print' into a PDF document. Have hardly printed (on paper) anything in the past 10 years.
If you are printing some material for use in the school then using one of their mchines and printer would be an obvious solution, would not get you involved in learning their security methods and may cause you (and their staff) far less head scratching!
(*) using a USB stick you could have school-related and pupil-related material information and be more easily able to keep it separate from the computer, so having the computer stolen would be a loss, but would not include information about lots of schools (eg wireless access) or about individual pupils, or (worse) whole classes of pupils.