My advice is, do not go with thin clients. If budget is an issue go with Asus Eee boxes or a Shuttle SSF PC. They can come down in price to a level comparable with a thin client. Yet they are poweful enough for school purposes.IF necessary they can act as thin clients anyways.
Some departments yes, will need more powerful computers. The students who will do any work on autocad, photoshop, video editing will need more juice. If you are going with Windows, I doubt you can still license XP. But you can look into Windows FLP (for legacy PCs). If you go this way, one interesting thing you can do is put a Linux box in every class (or every compsci class) with several different distros that students can play around with ( ubuntu desktop, slackware, CentOS server).
I am not sure about handing out tablets. Do the students really need mobility. Having a few laptops and tablets(only 10 or 20) that can be borrowed when needed is good,i.e. students working on programming a small robot; but expecting everyone, students, staff and school-board, to take advantage of the opportunity if every student has a tablet is unrealistic.