mac to pc office files and vice versa should be fully compatible providing you are both using similar version products - mac office 2004 should read any file created in pc office providing it isnt a new 2007 file. mac office 2008 is out soon to make that bridge. as was said - archive your attachment into a zip file to mail it to avoid data corruption - a mac can open zip files as standard.
Stuffit Expander is not default OS X software anymore. Zip will work - though it sometimes does odd things to custom icons, but that shouldn't be an issue for a spreadsheet.
.xls should open fine if your teacher has Office or OpenOffice of a suitable version. If it doesn't, try using "save as" and choose an _earlier_ version of Excel. That sometimes works wonders. I have no trouble exchanging files with Windows Excel users and I use OpenOffice. (Word did used to cause problems, but that was ages ago.)
Otherwise, if you don't need the teacher to edit it, can you turn the spreadsheet into a PDF?
Looking at the dates, I suspect you either sorted it or not, by now, but still...
I have a pc, he has a mac. What kindof .*** should i save it as? .xls doesn't work (well at least on the mac i tried it on a week ago).
Thanks.