YES, you CAN! 
YES, you can do this! There are several apps out there, however, one or two are the best. One is called Crazy Remote Pro and only works with PC, not Mac, claims to be faster, but new for the iPad (previously had iPhone/iPod touch only version) for $20. The other is called LogMeIn Ignition, it works with PC or Mac, so you could use at home if you wanted. It costs $30, but isn't as new, might not be as fast. (another app I already mentioned TeamViewer does much the same thing PC or Mac, but is slow and clunky, however, free).
These apps all work pretty much the same way, you install something on your desktop/laptop and set up an account. You also install the iPad app and sign in, and you can log into either one (except Crazy Remote Pro only works w/PC). You can be anywhere with the iPad as it connects via the internet. I don't know about the other two apps, I assume they also work this way, but with TeamViewer you have to either leave your computer on and turn off sleep mode, or change the settings in your computer so that it will wake from sleep when TeamViewer is accessed.
It does not matter what software you want to use, what these apps do is turn your iPad into a remote for your computer, not like your TV's remote, but your iPad will access your desktop and you will see your desktop on the iPad screen and you can control your computer remotely in this way, as if you were sitting in front of your desktop. As long as you have wifi in your classroom you can go anywhere in your classroom with the iPad and control your PC w/the touchscreen. Wherever you get internet access w/your iPad you can use one of these programs to control your computer. You may need to check with your IT dept. (if you have one) as far as any special set-up of your school's wifi internet access. I personally am not familiar with office or school wifi/internet networks, just home networks.
So, one of these apps is all you need, unless I misunderstand you and you need to be able to actually get something from your PC onto your iPad. If you are able to sit at your desk and work on your PC while a presentation is playing (I assume via a projector), then you can do the same with an iPad using one of these apps. Just keep in mind, you are not transferring anything, just remotely accessing and controlling your PC.
http://www.crazyremote.com/crazyremote/eng/index.html
https://secure.logmein.com/
http://www.youtube.com/watch?v=Qn2wM3WPztg
(this will be too slow for you and I don't recommend, but it is free: http://www.teamviewer.com/en/index.aspx)
Does this answer your question?
I luv my iPad and am a bit bored right now .
I wrote all the above before the news about ipad2...nothing significant changes for what you want to do except maybe two things: iPad2 has HDMI out, you could conceivably use the iPad2 with Keynote for presentations, however, you wouldn't be able to use it for grading or other things at the same time. Although, I would recommend getting the iPad2 over the iPad, yes, I understand the iPad is $100 less on clearance, however, for what you want to do with it, the increased power of the new dual-core CPU in the iPad2 will be an advantage you won't want to pass up!
YES, you can do this! There are several apps out there, however, one or two are the best. One is called Crazy Remote Pro and only works with PC, not Mac, claims to be faster, but new for the iPad (previously had iPhone/iPod touch only version) for $20. The other is called LogMeIn Ignition, it works with PC or Mac, so you could use at home if you wanted. It costs $30, but isn't as new, might not be as fast. (another app I already mentioned TeamViewer does much the same thing PC or Mac, but is slow and clunky, however, free).
These apps all work pretty much the same way, you install something on your desktop/laptop and set up an account. You also install the iPad app and sign in, and you can log into either one (except Crazy Remote Pro only works w/PC). You can be anywhere with the iPad as it connects via the internet. I don't know about the other two apps, I assume they also work this way, but with TeamViewer you have to either leave your computer on and turn off sleep mode, or change the settings in your computer so that it will wake from sleep when TeamViewer is accessed.
It does not matter what software you want to use, what these apps do is turn your iPad into a remote for your computer, not like your TV's remote, but your iPad will access your desktop and you will see your desktop on the iPad screen and you can control your computer remotely in this way, as if you were sitting in front of your desktop. As long as you have wifi in your classroom you can go anywhere in your classroom with the iPad and control your PC w/the touchscreen. Wherever you get internet access w/your iPad you can use one of these programs to control your computer. You may need to check with your IT dept. (if you have one) as far as any special set-up of your school's wifi internet access. I personally am not familiar with office or school wifi/internet networks, just home networks.
So, one of these apps is all you need, unless I misunderstand you and you need to be able to actually get something from your PC onto your iPad. If you are able to sit at your desk and work on your PC while a presentation is playing (I assume via a projector), then you can do the same with an iPad using one of these apps. Just keep in mind, you are not transferring anything, just remotely accessing and controlling your PC.
http://www.crazyremote.com/crazyremote/eng/index.html
https://secure.logmein.com/
http://www.youtube.com/watch?v=Qn2wM3WPztg
(this will be too slow for you and I don't recommend, but it is free: http://www.teamviewer.com/en/index.aspx)
Does this answer your question?
I luv my iPad and am a bit bored right now .
I wrote all the above before the news about ipad2...nothing significant changes for what you want to do except maybe two things: iPad2 has HDMI out, you could conceivably use the iPad2 with Keynote for presentations, however, you wouldn't be able to use it for grading or other things at the same time. Although, I would recommend getting the iPad2 over the iPad, yes, I understand the iPad is $100 less on clearance, however, for what you want to do with it, the increased power of the new dual-core CPU in the iPad2 will be an advantage you won't want to pass up!