what's the best way to do it? go to my bank where the money is, and tell them I want to make a withdrawal and exchange at the same time? then hold on to the canadian cash in a safety deposit box? Or is there a way to do the exchange on paper and just keep it all still in the bank?"Hoarding" cash is dumb. An easy thing you can do which gives to market the most information possible, is to buy a collection of Canadian Government bonds which expire at the beginning of each semester or whenever you anticipate needing the money. You can check out the C$ yield curve here:
http://www.bankofcanada.ca/en/rates/yield_curve.htmlSo let's say for example for your son's senior year in 3 years you can anticipate him needing 10000CAD, and the yield for 3 year zero interest bonds is 3.5%. Then you can buy bonds which will provide that 10000 and they'll cost you (1.035)^-3 *10000CAD = 9019.42CAD. So you save ~10% over putting cash in a safety deposit box.
Now there are tons of other ways (forex,futures,options,unit funds) you can do the same thing, but I'm pretty sure any transaction where you pay in USD now for locked in amount of CAD later you're going to get about the same deal. The only thing that can change will be the transaction costs. Some of the more advanced ways of doing this like Forex or Futures trading cost you too much.
Ask yourself the following:
1) Do you have all the money now, or would you like to lock in the exchange rate of USD you know you'll have in the future? You can enter forward trading agreements, where you agree to exchange X USD for Y CAD at time T.
2) Do you want to play the private sector? You can probably get a higher rate of return if you put the money in the Canadian private sector, for example you could buy corporate bonds with higher yields that also pay out in CAD.
3) Do you want to play the exchange rate? Are you very sure the USD is going down relative to CAD? You can bet hard against the USD and wind up loosing money if you're wrong, or you could bet light against the USD and buy some options to protect you from huge drops in the dollar.