Well, page 13 of Apple's 2005 10-K states that over 12% of net sales were through its US education channel. Unfortunately, Apple doesn't provide a product breakdown of its education sales, so you can't tell how much is software, Macs, iPods, etc.
The best idea, as olePigeon suggests, is to find Apple's education market share, and then use the unit sales by operating segment to find the number of Macs. Note that, if you want US numbers, Apple doesn't provide US unit sales, but it does provide unit sales for the Americas. I imagine you can estimate the US unit sales by using Apple's US vs. non-US sales numbers, but I don't know if Apple breaks that down by division or product either, so it probably reflects iPod sales, etc.
And, if you want Apple's total education sales, well, you'll either have to find worldwide education market share to use with total units sold, or find an overall education sales figure/breakdown somewhere in Apple's report, which I doubt it provides.